BERKELEY 
 
 LIBRARY 
 
 UNIVERSITY OP 
 CALIFORNIA 
 
 EARTH 
 
 SCIENCES 
 
 LIBRARY 
 


 
 THE INTERNATIONAL SCIENTIFIC SERIES. 
 VOLUME XXXV. 
 
INTERNATIONAL SCIENTIFIC SERIES. 
 
 NOW READY. In 12mo, and bound in cloth. 
 
 1. FORMS OF WATER. By Prof. JOHN TYNDALL. $1.50. 
 
 2. PHYSICS AND POLITICS. By W. BAGEHOT. $1.50. 
 
 3. FOODS. By EDAVARD SMITH, M. D., LL. B., F. R. S. $1.75. 
 
 4. MIND AND BODY. By ALEXANDER BAIN, LL. D. $1.50. 
 
 5. THE STUDY OF SOCIOLOGY. By HERBERT SPENCER. $1.50. 
 
 6. THE NEW CHEMISTRY. By Prof. JOSIAH P. COOKE, Jr. $2.00. 
 
 7. THE CONSERVATION OF ENERGY. By Prof. BALFOUR STEWART. $1.50. 
 
 8. ANIMAL LOCOMOTION. By J. B. PETTIGREW, M.D. $1.75. 
 
 9. RESPONSIBILITY IN MENTAL DISEASE. By H. MAVDSLEY, M.D. 
 
 $1.50. 
 
 10. THE SCIENCE OF LAW. By Prof. SHELDON AMOS. $1.75. 
 
 11. ANIMAL MECHANISM. By E. J. MAREY.' 117 Illustrations. $1.75. 
 
 12. CONFLICT BETWEEN RELIGION AND SCIENCE. By J. W. DRAPER, 
 
 M.D. $1.75. 
 
 13. THE DOCTRINE OF DESCENT, AND DARWINISM. By Prof. O. SCHMIDT. 
 
 $1.50. 
 
 14. THE CHEMISTRY OF LIGHT AND PHOTOGRAPHY. By Dr. HERMANN 
 
 VOGEL. 100 Illustrations. $2.00. 
 
 15. FUNGI ; their Nature, Influence, and Uses. By M. C. COOKE, LL. D. Edited 
 
 by M. J. BERKELEY. 109 Illustrations. $1.50. 
 
 16. THE LIFE AND GROWTH OF LANGUAGE. By Prof. W. D. WHITNEY, of 
 
 Yale College. $1.50. 
 
 17. MONEY AND THE MECHANISM OF EXCHANGE. By W. STANLEY 
 
 JEVONS, M. A., F. R. S. $1.75. 
 
 18. THE NATURE OF LIGHT. By Dr. E. LOMMEL. 88 Illustrations and a Plate 
 
 of Spectra in Chromo-lithography. $2.00. 
 
 19. ANIMAL PARASITES AND MESSMATES. By Prof. M. VAN BENEDEN. 
 
 83 Illustrations. $1.50. 
 
 20. ON FERMENTATIONS. By P. SCHUTZENBERGEK. $1.50. 
 
 21. THE FIVE SENSES OF MAN. By Prof. J. BERNSTEIN. $1.75. 
 
 22. THE THEORY OF SOUND IN ITS'RELATION 10 MUSIC. By Prof. PIKTRO 
 
 BLASERNA. Numerous Woodcuts. $1.50. 
 
 23. STUDIES IN SPECTRUM ANALYSIS. By J. N. LOCKYER. $2.50. 
 
 24. A HISTORY OF THE GROWTH OF THE STEAM-ENGINE. By ROBERT 
 
 H. THURSTON, A.M., C. E. 163 Illustrations. $2.50. 
 
 25. EDUCATION AS A SCIENCE. By ALEX. BAIN, LL.D. $1.75. 
 
 26. MODERN CHROMATICS. By Prof. 0. N. ROOD. 130 original Illustrations. $2.00. 
 
 27. THE HUMAN SPECIES. By A. DE QUATPEFAGES. *'2.00. 
 
 28. THE CRAYFISH : An Introduction to the Study of Zoology. By Prof. T. H. 
 
 HUXLEY. 82 Illustrations. $1.75. 
 
 29. THE ATOMIC THEORY. By AD. WURTZ, Membre de Tlnstitut, etc. Trans- 
 
 lated by E. CLEMINSHAAV. $1.50. 
 
 30. ANIMAL LIFE AS AFFECTED BY THE NATURAL CONDITIONS OF 
 
 EXISTENCE. By Prof. KARL SEMPER. $2.00. 
 
 31. SIGHT : An Exposition of the Principles of Monocular and Binocular Vision. By 
 
 JOSEPH LE COXTE, LL. D. With numerous Illustrations. $1.50. 
 
 32. GENERAL PHYSIOLOGY OF MUSCLES AND NERVES. By Dr. I. ROSEN 
 
 THAL. With 75 Woodcuts. $1.50. 
 
 33. ILLUSIONS : A Psychological Study. By JAMES SULLY. $1.50. 
 
 34. THE SUN. By C. A. YOUNG, Ph. D., LL. I)., Professor of Astronomy in the ! 
 
 College of New Jersey. With numerous Illustrations. $2.00. 
 
 Any volume sent by mail, post-paid, on receipt of price. 
 
 NEW YORK: D. APPLETON & CO., 1, 3, & 5 BOND STREET. 
 
Frontispiece 
 
 SECTIONS OF IGNEOUS ROCKS, ILLUSTRATING THE PASSAGE FROM THE 
 GLASSY TO THE CRYSTALLINE STRUCTURE. 
 
 Vitreous Bock. 2. Semi- Vitreous Rock. 3. Vitreous Hock with Sphserulites. 4. Rock with 
 Cryptocrystalline Base. 5. Rock with Microcrystalline Base. 6. Rock of Granitic Structure 
 built up entirely of Crystals. [See pp. 53-58. 
 
THE INTERNATIONAL SCIENTIFIC SERIES. 
 
 VOLCANOES : 
 
 WHAT THEY ARE AND WHAT THEY TEACH. 
 
 BY 
 
 JOHN W. JUDD, F. R. S., 
 
 \N 
 PROFESSOR OF GEOLOGY IN THE ROYAL SCHOOL OF MINES. 
 
 WITH NINETY-SIX ILLUSTRATIONS. 
 
 NEW YORK: 
 D. APPLETON AND COMPANY, 
 
 1, 3, AND 5 BOND STEEET. 
 
 1881. 
 
-.v / 
 

 
 PREFACE. 
 
 IN PREPARING THIS WORK, I have aimed at carrying out 
 a design suggested to me by the late Mr. Poulett 
 Scrope, the accomplishment of which has been un- 
 fortunately delayed, longer than I could have wished, 
 by many pressing duties. 
 
 Mr. Scrope's well-known works, entitled 'Volcanoes' 
 and 'The Greology and Extinct Volcanoes of Central 
 France ' which passed through several editions in this 
 country, and have been translated into the principal 
 European languages embody the results of much 
 careful observation and acute reasoning -upon the 
 questions which the author made the study of his life. 
 In the first of these works the phenomena of volcanic 
 activity are described, and its causes discussed; in the 
 second it is shown that much insight concerning these 
 problems may be obtained by a study of the ruined and 
 denuded relics of the volcanoes of former geological 
 periods. The appearance of these works, in the years 
 
 92081 
 
vi PJBEFACE. 
 
 1825 and 1827 respectively, did much to prepare the 
 minds of the earlier cultivators of science for the 
 reception of those doctrines of geological uniformity 
 and continuity, which were shortly afterwards so ably 
 advocated by Lyell in his ' Principles of Geology.' 
 
 Since the date of the appearance of the last editions 
 of Scrope's works, inquiry and speculation concerning 
 the nature and -origin of volcanoes have been alike 
 active, and many of the problems which were discussed 
 by him, now present themselves under aspects entirely 
 new and different from those in which he was accus- 
 tomed to regard them. No one was ever more ready 
 to welcome original views or to submit to having long- 
 cherished principles exposed to the ordeal of free 
 criticism than was Scrope; and few men retained to 
 so advanced an age the power of subjecting novel 
 theories to the test of a rigorous and logical com- 
 parison with ascertained facts. 
 
 But this eminent geologist was not content with 
 the devotion of his own time and energies to the 
 advancement of his favourite science, for as increas- 
 ing age and growing infirmities rendered travel and 
 personal research impossible, he found a new source of 
 pleasure in seeking out the younger workers in those 
 fields of inquiry which he had so long and successfully 
 cultivated, and in furthering their efforts by his judicious 
 
PKEFACE. vii 
 
 advice and kindly aid. Among the chosen disciples of 
 this distinguished man, who will ever be regarded as 
 one of the chief pioneers of geological thought, I had 
 the good fortune to be numbered, and when he com- 
 mitted to me the task of preparing a popular exposi- 
 tion of the present condition of our knowledge on 
 volcanoes, I felt that I had been greatly honoured. 
 
 In order to keep the work within the prescribed 
 limits, and to avoid unnecessary repetitions, I have 
 confined myself to the examination of such selected 
 examples of volcanoes as could be shown to be really 
 typical of all the various classes which exist upon the 
 globe ; and I have endeavoured from the study of these 
 to deduce those general laws which appear to govern 
 volcanic action. But it has, at the same time, been 
 my aim to approach the question from a somewhat 
 new standpoint, and to give an account of those in- 
 vestigations which have in recent times thrown so 
 much fresh light upon the whole problem. In this 
 way I have been led to dwell at some length upon 
 subjects which might not at first sight appear to be. 
 germane to the question under discussion ; such as 
 the characters of lavas revealed to us by microscopic 
 examination ; the nature and movements of the liquids 
 enclosed in the crystals of igneous rocks ; the relations 
 of minerals occurring in some volcanic products to 
 those found in meteorites ; the nature and origin of 
 
viii PREFACE. 
 
 the remarkable iron-masses found at Ovifak in Green- 
 land ; and the indications which have been discovered 
 of analogies between the composition and dynamics of 
 our earth and those of other members of the family of 
 worlds to which it belongs. While not evading the 
 discussion of theoretical questions, I have endeavoured 
 to keep such discussions in strict subordination to that 
 presentation of the results attained by observation and 
 experiment, which constitutes the principal object of 
 the work. 
 
 The woodcuts which illustrate the volume are in 
 some cases prepared from photographs, and I am in- 
 debted to Mr. Cooper for the skill with which he has 
 carried out my wishes concerning their reproduction. 
 Others among the engravings are copies of sketches 
 which I made .in Italy, Hungary, Bohemia, and other 
 volcanic districts. The whole of the wood-blocks em- 
 ployed by Mr. Poulett Scrope in his work on Volcanoes 
 were placed at my disposal before his death, and such 
 of them as were useful for my purpose I have freely 
 employed. To Captain S. P. Oliver, R.A., I am obliged 
 for a beautiful drawing made in the Island of Bourbon, 
 and to Mr. Norman Lockyer and his publisher s Messrs. 
 Macmillan & Co., for the use of several wood-blocks 
 illustrating sun-spots and solar prominences. 
 
 J. W. J. 
 LONDON: .May 1881. 
 
CONTENTS. 
 
 CHAPTER I. 
 
 PACUS 
 INTRODUCTORY : NATURE OF THE ENQUIRY .... 1 
 
 CHAPTER II. 
 
 THE NATURE OF VOLCANIC ACTION 7 
 
 CHAPTER III. 
 
 THE PRODUCTS OF VOLCANIC ACTION 39 
 
 CHAPTER IY. 
 
 THE DISTRIBUTION OF THE MATERIALS EJECTED FBOM 
 
 VOLCANIC VENTS 67 
 
 CHAPTER Y. 
 
 THE INTERNAL STRUCTURE OF VOLCANIC MOUNTAINS . .112 
 
 CHAPTER YI. 
 
 THE VARIOUS STRUCTURES BUILT UP AROUND VOLCANIC 
 
 VENTS 151 
 
 CHAPTER VII. 
 
 THE SUCCESSION OF OPERATIONS TAKING PLACE AT VOL- 
 CANIC CENTRES 186 
 
X CONTENTS 
 
 CHAPTER VIII. 
 
 PAGE 
 THE DISTRIBUTION OF VOLCANOES UPON THE SURFACE OF 
 
 THE GLOBE . . . 224 
 
 CHAPTER IX. 
 
 VOLCANIC ACTION AT DIFFERENT PERIODS OF THE EARTH'S 
 
 HISTORY 247 
 
 CHAPTER X. 
 
 THE PART PLAYED BY VOLCANOES IN THE ECONOMY OF 
 
 NATURE 281 
 
 CHAPTER XL 
 
 WHAT VOLCANOES TEACH US CONCERNING THE NATURE OF 
 
 THE EARTH'S INTERIOR 307 
 
 CHAPTER XII. 
 
 THE ATTEMPTS WHICH HAVE BEEN MADE TO EXPLAIN THE 
 
 CAUSES OF VOLCANIC ACTION 331 
 
 INDEX . 371 
 
ILLUSTRATIONS. 
 
 Sections of igneous rocks illustrating the passage from the 
 
 glassy to the crystalline structure . . . Frontispiece 
 
 FIG. PAGB 
 
 1. Stromboli, viewed from the north-west, April 1874 to face p. 10 
 
 2. Map of the Island of Stromboli 11 
 
 3. Section through the Island of Stromboli from north-west to 
 
 south-east 13 
 
 4. The crater of Stromboli as viewed from the side of the 
 
 Sciarra during an eruption on the morning of April 24, 
 1874 14 
 
 6. Vesuvius in eruption, as seen from Naples, April 26, 1872. 
 
 (From a photograph} to face p. 24 
 
 6. View of Vulcano, with Vulcanello in the foreground taken 
 
 from the south end of the Island of Lipari . . .43 
 
 7. Minute cavities, containing liquids, in the crystals of rocks. 
 
 (After Zirkel} to face p. 60 
 
 8. Minute liquid-cavity in a crystal, with a moving bubble. 
 
 (After Hartley} 62 
 
 9. Cavity in crystal, containing carbonic-acid gas at a tempe- 
 
 rature of 86 F., and passing from the liquid to the 
 gaseous condition. (After Hartley} . . . .64 
 
 10. Monte Nuovo (440 ft. high) on the shores of the Bay of 
 
 Naples. (After Scrope} .76 
 
 11. Map of the district around Naples, showing Monte Nuovo 
 
 and the surrounding volcanoes of glder date . . .78 
 
xii ILLUSTRATIONS. 
 
 FIG. I'AGK 
 
 12. Outlines of the summit of Vesuvius during the eruption of 
 
 1767. (After Sir W, Hamilton) . . . to face p. 80 
 
 13. Crater of Vesuvius formed during the eruption of 1822 
 
 (After Scrape) . .82 
 
 14. Crater of Vesuvius in 1756, from a drawing made on the 
 
 spot. (After Sir W. Hamilton) 84 
 
 15. The summit of Vesuvius in 1767, from an original drawing. 
 
 (After Sir W. Hamilton) 85 
 
 16. Summit of Vesuvius in 1843 86 
 
 17. Outlines of Vesuvius, showing its form at different periods 
 
 of its history 87 
 
 18. Cascade of lava tumbling over a cliff in the Island of 
 
 Bourbon. (After Capt. 8. P. Oliver, R.A.) . . .93 
 
 19. Lava-stream (obsidian) in the Island of Vulcano, showing 
 
 the imperfect liquidity of the mass .... 95 
 
 20. Interior of a rhyolitic lava-stream in the Island of Lipari, 
 
 showing broad, sigmoidal folds, produced by the slow 
 movements of the mass . 96 
 
 21. Interior of a rhyolitic lava-stream in the Island of Lipari, 
 
 showing the complicated crumplings and puckerings, 
 produced by the slow movements of the mass . . 96 
 
 22. Vesuvian lava-stream of 1858, exhibiting the peculiar 
 
 * ropy ' surfaces of slowly-moving currents. (From a 
 photograph) to face p. 98 
 
 23. Vesuvian lava-stream of 1872, exhibiting the rough cindery 
 
 surfaces characteristic of rapidly flowing currents. 
 (From a, photograph} to face p. 98 
 
 24. Concentric folds on mass of cooled lava. (After Heaphy) . 100 
 
 25. Mass of cooled lava formed over a spiracle on the slopes 
 
 of Hawaii. (After Dana) 100 
 
 26. Group of small cones thrown up on the Vesuvian lava-cur- 
 
 rent of 1855. (After Schmidt) ... .101 
 
 27. Natural section of a lava-stream in the Island of Vulcano, 
 
 showing the compact central portion and the scoriaceous 
 upper and under surfaces 104 
 
 28. Section of a lava-stream, exposed on the side of the river 
 
 Ardeche, in the south-west of France. (After Scrope) . 106 
 
ILLUSTKATIONS. xiii 
 
 FIG. PAGE 
 
 29. Portion of a basaltic column from the Giant's Causeway, 
 
 exhibiting both the ball-and-socket and the tenon-and- 
 mortise structure 107 
 
 30. Vein of green pitchstone at Chiaja di Luna, in the Island 
 
 of Ponza, breaking up into regular columns and into 
 spherical masses with a concentric series of joints. 
 (After Scrape) 108 
 
 31. Illustration pf the 'perlitic structure ' in glassy rocks . 109 
 
 32. Transverse section of a lava-stream Ill 
 
 33. The Kammerbiihl, or Kammerberg, Bohemia (as seen from 
 
 the south-west) 113 
 
 34. Section of the Kammerbiihl in Bohemia . . . .lit 
 
 35. Natural section of a volcanic cone in the Island of 
 
 Vulcano 116 
 
 36. Section in the side of the Kammerbiihl, Bohemia . .118 
 
 37. Experimental illustration of the mode of formation of 
 
 volcanic cones, composed of fragmental materials . 120 
 
 38. Natural section of a tuff -cone, forming the Cape of 
 
 Misenum, and exhibiting the peculiar internal arrange- 
 ment, characteristic of volcanoes composed of frag- 
 mentary materials. {After Scrape") .... 121 
 
 39. Section of a small scoria-cone formed within the crater of 
 
 Vesuvius in the year 1835, illustrating the filling up of 
 the central vent of the cone by subsequent ejections. 
 (After AUcJi) 122 
 
 40. Volcanic cones composed of scorise, and breached on one 
 
 side by the outflow of lava-currents. (After Scrape} . 123 
 
 41. Campo Bianco, in the Island of Lipari. A pumice-cone 
 
 breached by the outflow of an obsidian lava-stream 
 
 to face p. 124 
 
 42. Volcanic cones in Auvergne, which have suffered to some 
 
 extent from atmospheric denudation. (After Scrap ^ . 124 
 
 43. Experimental illustration of the mode of formation of 
 
 volcanic cones composed of viscid lavas. ( After lleyer) 126 
 
 44. The Grand Puy of Sarcoui, composed of trachyte, rising 
 
 between two breached scoria-cones (Auvergne). (After 
 Scrape) 120 
 
XIV ILLUSTRATIONS. 
 
 FIG. PAGE 
 
 45. Volcanic cone (Mamelon) composed of very viscid lava 
 
 (Island of Bourbon). {After Bory de St. Vincent} . 127 
 
 46. Another Mamelon in the Island of Bourbon, with a crater 
 
 at its summit. {After Bory de St. Vincent) . . .127 
 
 47. Cliff-section in the Island of Madeira, showing how a com- 
 
 posite volcano is built up of lava-streams, beds of scorias, 
 
 and dykes. (After Lye II) 128 
 
 48. Section seen at the cascade, Bains de Mont Dore. {After 
 
 Scrope) 130 
 
 49. Section in the Island of Ventotienne, showing a great 
 
 stream cf andesitic lava overlying stratified tuffs. 
 (After Scrope) 130 
 
 50. Cliff on the south side of the Island of San Stephano . 131 
 
 51. The headland of Monte della Gruardia, in the Island of 
 
 Ponza 131 
 
 52. Western side of the same headland, as seen from the north 
 
 side of Luna Bay 132 
 
 53. Sea-cliff at II Capo, the north-east point of Salina, show- 
 
 ing stratified agglomerates traversed by numerous dykes, 
 the whole being unconformably overlaid by stratified, 
 aqueous deposits 132 
 
 54. Section observed in the Val del Bove, Etna, showing a 
 
 basaltic dyke, from the upper part of which a lava- 
 current has flowed 133 
 
 55. Basaltic dykes projecting from masses of stratified scorise 
 
 in the sides of the Val del Bove, Etna .... 134 
 
 56. Sheets of igneous rock (basalt) intruded between beds of 
 
 sandstone, clay, and limestone (Island of Skye) . . 137 
 
 57. Plan of the dissected volcano of Mull in the Inner 
 
 Hebrides to face p. 142 
 
 58. Section of the volcano of Mull along the line A B 142 
 
 59. Summit of the volcano of Monte Sant' Angelo, in Lip^ri, 
 
 exhibiting a crater with walls worn down by denuua- 
 tion . . 158 
 
 60. Outlines of lava-cones . 160 
 
 61. Diagram illustrating the formation of parasitic cones along 
 
 lines of fissure formed on the flanks of a great volcanic 
 mountain 162 
 
ILLUSTRATIONS. XV 
 
 FIG. 1'AGK 
 
 62. Outline of Etna, as seen from Catania .... 162 
 
 63. Outline of Etna, as seen from the Val del Bove . . 163 
 
 64. Plan of the volcano forming the Island of Ischia . . 16S 
 
 65. A primary parasitic cone with a secondary one at its base 
 
 Ischia .164 
 
 66. Scoria-cone near Auckland, New Zealand, with a lava- 
 
 current flowing from it. (After HeapJiy) . . .165 
 
 67. Section of rocks below the ancient triassic volcano of 
 
 Predazzo in the Tyrol 165 
 
 68. Cotopaxi, as seen from a distance of ninety miles. (After 
 
 Humboldt) 168 
 
 69. Citlaltepetl, or the Pic d'Orizaba, in Mexico, as seen from 
 
 the Forest of Xalapa. (After. Huniboldt") . . .169 
 
 70. Lac Paven, in the Auvergne. (After Scrope) . . .171 
 
 71. The crater-lake called Lagodel Bagno, in Ischia, converted 
 
 into a harbour 172 
 
 72. Lake of Gustavila, in Mexico. (After Humboldt) . . 172 
 
 73. Peak of Teneriffe, surrounded by great crater-rings. (After 
 
 Piazzi-SmytJi) 175 
 
 74. The volcano of Bourbon, rising in the midst of a crater- 
 
 ring four miles in diameter. (After Bory de St. 
 Vincent) 176 
 
 75. The volcano of Bourbon, as seen from another point of 
 
 view, with three concentric crater-rings encircling its 
 base. (After JBory de St. Vincent) . . . .176 
 
 76. Vesuvius as seen from Torrento, half encircled by the 
 
 crater-ring of Somma 177 
 
 77. Outlines of various volcanoes illustrating the different 
 
 relations of the craters to cones . . . to face p. 178 
 
 78. Island thrown up in the Mediterranean Sea in July and 
 
 August, 1831. (After the Prince de Joinville) . .179 
 
 79. Sinter-cones surrounding the orifices of geysers . . .183 
 
 80. Diagram illustrating the mode of formation of travertine- 
 
 and sinter-terraces on the sides of a hill of tuff . .185 
 
 81. Map of the volcanic group of the Lipari Islands, illus- 
 
XVi ILLUSTRATIONS. 
 
 FIG. PAGB 
 
 trating the position of the lines of fissure upon which 
 
 the volcanoes have been built up . . . ... 192 
 
 82. The Puy de Pariou, in the Auvergne, illustrating the shift- 
 
 ing of eruption along a line of fissures . . . .193 
 
 83. Ideal section of the Puy de Pariou 194 
 
 84. Fissure formed on the flanks of Etna during the eruption 
 
 of 1865. (After Silvertri) 194 
 
 85. Plan of the Island of Vulcano, based on the map of the 
 
 Italian Government 195 
 
 86. Vulcanello, with its three craters 197 
 
 87. Section of basalt from Ovifak, Greenland, with particles of 
 
 metallic iron diffused through its mass . . . .319 
 
 88. Diagram illustrating the relations between the terrestrial 
 
 and the extra-terrestrial rocks . . . tofatwp.322 
 
 89. A group of sun-spots. (After Seecki) . . . .362 
 
 90. A sun-spot, showing the great masses of incandescent 
 
 vapour rising or falling within it. (After Se.cc?ii) . . 363 
 
 91. The edge of a sun-spot, showing a portion of the promi- 
 
 nent masses of incandescent gas (A) which detached 
 itself at E and floated into the midst of the cavity. 
 (After Norman Lockyer) 363 
 
 92. Drawing of a solar prominence made by Mr. Norman 
 
 Lockyer, March 14, 1869, at 11 h. 5m. A.M. . . .364 
 
 93. The same object, as seen at llh. 15m. on the same day. 
 
 (After Norman Lockyer} 365 
 
 94. Drawings of a solar prominence at four different periods 
 
 on September 7, 1871. (After Young") .... 366 
 
 95. A group of Lunar craters (Maurolycus, Barocius, &c.), the 
 
 largest being more than sixty miles in diameter . . 368 
 
VOLCANOES. 
 
 CHAPTER I. 
 
 INTRODUCTORY: NATURE OF THE INQUIRY. 
 
 ' WHAT is A VOLCANO?' This is a familiar question, 
 often addressed to us in our youth, which 4 Catechisms 
 of Universal Knowledge,' and similar school manuals, 
 have taught us to reply to in some such terms as 
 the following : c A volcano is a burning mountain, from 
 the summit of which issue smoke and flames.' Such 
 a statement as this, it is probable, does not unfairly 
 represent the ideas which are, even at the present day, 
 popularly entertained upon the subject. 
 
 But in this, as in so many other cases, our first 
 step towards the acquirement of scientific or exact 
 knowledge, must be the unlearning of what we have 
 before been led to regard as true. The description 
 which we have quoted is not merely incomplete and 
 inadequate as a whole, but each individual proposition 
 of which it is made up is grossly inaccurate, and, what 
 
2 VOLCANOES. 
 
 is worse, perversely misleading. In the first place, the 
 action which takes place at volcanoes is not ' burning,' 
 or combustion, and bears, indeed, no relation whatever 
 to that well-known process. Nor are volcanoes neces- 
 sarily mountains ' at all ; essentially, they are just 
 the reverse namely, holes in the earth's crust, or outer 
 portion, by means of which a communication is kept 
 up between the surface and the interior of our globe. 
 When mountains do exist at centres of volcanic ac- 
 tivity, they are simply the heaps of materials thrown 
 out of these holes, and must therefore be regarded not 
 as the causes but as the consequences of the volcanic 
 action. Neither does this action always take place at 
 the ' summits ' of volcanic mountains, when such exist, 
 for eruptions occur quite as frequently on their sides 
 or at their base. That, too, which popular fancy regards 
 as ' smoke ' is really condensing steam or watery vapour, 
 and the supposed raging ' flames ' are nothing more 
 than the glowing light of a mass of molten material 
 reflected from these vapour clouds. 
 
 It is not difficult to understand how these false 
 notions on the subject of volcanic action have come to 
 be so generally prevalent. In the earlier stages of its 
 development, the human mind is much more con- 
 genially employed in drinking in that which is marvel- 
 lous than in searching for that which is true. It must 
 be admitted, too, that the grand and striking pheno- 
 mena displayed by volcanoes are especially calculated 
 to inspire terror and to excite superstition, and such 
 
IDEAS OF THE ANCIENTS. 3 
 
 feelings must operate in preventing those close and 
 accurate observations which alone can form the basis 
 of scientific reasoning. 
 
 The ancients were acquainted only with the four or 
 five active volcanoes in the Mediterranean area; the 
 term ' volcano ' being the name of one of these (Vul- 
 cano, or Volcano, in the Lipari Islands), which has 
 come to be applied to all similar phenomena. It is 
 only in comparatively modern times that it has become 
 a known fact that many hundreds of volcanoes exist 
 upon the globe, and are scattered over almost every 
 part of its surface. Classical mythology appropriated 
 Vulcano as the forge of Hephaestus, and his Roman 
 representative Vulcan, while Etna was regarded as 
 formed by the mountains under which the vengeful 
 deity had buried the rebellious Typhon; it may be 
 imagined, therefore, that any endeavour to more 
 closely investigate the phenomena displayed at these 
 localities would be regarded, not simply as an act of 
 temerity, but as one of actual impiety. In mediaeval 
 times similar feelings would operate with not less 
 force in the same direction, for the popular belief 
 identified the subterranean fires with a place of ever- 
 lasting torment ; Vulcauo was regarded as the place of 
 punishment of the Arian Emperor Theodosius, while 
 Etna was assigned to poor Anne Boleyn, the perverter 
 of faith in the person of its stoutest defender. That 
 such feelings of superstitious terror in connection with 
 volcanoes are, even at the present day, far from being 
 
4 VOLCANOES. 
 
 extinct, will be attested by every traveller who, in 
 carrying on investigations about volcanic centres, has 
 had to avail himself of the assistance of guides and 
 attendants from among the common people. 
 
 Among the great writers of antiquity we find 
 several who had so far emancipated their minds from 
 the popular superstitions as to be able to enunciate 
 just and rational views upon the subject of volcanoes. 
 Until quite recent times, however, their teaching was 
 quite forgotten or neglected, and the modern science 
 of Voilcanology may be said to have entirely grown up 
 within the last one hundred years. 
 
 The great pioneer in this important branch of re- 
 search was the illustrious Italian naturalist Spallanzani, 
 who, in the year 1788, visited the several volcanoes of 
 his native land, and published an account of the nu- 
 merous valuable and original observations which he had 
 made upon them. About the same time the French 
 geologist Dolomieu showed how much light might be 
 thrown on the nature of volcanic action, by a study of 
 the various materials which are ejected from volcanic 
 vents ; while our own countryman, Sir William Hamil- 
 ton, was engaged in a systematic study of the changes 
 in form of volcanic mountains, and of the causes 
 which determine their growth. At a somewhat later 
 date the three German naturalists, Von Buch, Hum- 
 boldt, and Abich, greatly extended our knowledge of 
 volcanoes by their travels in different portions of the 
 globe. 
 
OF MODERN BESEARCHES. 5 
 
 The first attempt, however, to frame a satisfactory 
 theory of volcanic action, and to show the part which 
 volcanoes have played in the past history of our globe, 
 together with their place in its present economy, was 
 made in 1825, by Poulett Scrope, whose great work, 
 6 Considerations on Volcanoes,' may be regarded as the 
 earliest systematic treatise on Vulcanology. Since the 
 publication of this work, many new lines of inquiry 
 have been opened up in connection with the subject, 
 and fresh methods of research have been devised and 
 applied to it. More exact observations of travellers 
 over wider areas have greatly multiplied the facts 
 upon which we may reason and speculate, and many 
 erroneous hypotheses which had grown up in connec- 
 tion with the subject have been removed by patient 
 and critical inquiry. 
 
 We propose in the following pages to give an out- 
 line of the present state of knowledge upon the subject, 
 and to indicate the bearings of those conclusions which 
 have already been arrived at, upon the great questions 
 of the history of our globe and the relations which 
 it bears to the other portions of the universe. In 
 attempting this task we cannot do better than take 
 up the several lines of inquiry in the order in which 
 they have been seized upon and worked out by the 
 original investigators ; for never, perhaps, is the de- 
 velopment of thought in the individual mind so natural 
 in its methods, and so permanent in its effects, as when 
 it obeys those laws which determined its growth in the 
 
6 VOLCANOES. 
 
 collective mind of the race. In our minds, as in our 
 bodies, development in the individual is an epitome, 
 or microcosmic reproduction, of evolution in the 
 species. 
 
CHAPTEE II. 
 
 THE NATURE OF VOLCANIC ACTION. 
 
 THE close investigation of what goes on within a 
 volcanic vent may appear at first sight to be a task 
 beset with so many difficulties and dangers that we 
 may be tempted to abandon it as altogether hopeless. 
 At the first recorded eruption of Vesuvius the elder 
 Pliny lost his life in an attempt to approach the 
 mountain and examine the action which was taking 
 place there ; and during the last great outburst of the 
 same volcano a band of Neapolitan students, whose 
 curiosity was greater than their prudence, shared the 
 same fate. 
 
 But in both these cases the inquirers paid the 
 penalty of having adopted a wrong method. If we 
 wish to examine the mode of working of a compli- 
 cated steam-engine, it will be of little avail for us to 
 watch the machinery when the full blast of steam is 
 turned on, and the rapid movements of levers, pinions, 
 and slides baffle all attempts to follow them, and render 
 hopeless every effort to trace their connection with 
 one another. But if some friendly hand turn off the 
 
8 VOLCANOES. 
 
 greater part of the steam-supply, then., as the rods 
 move slowly backwards and forwards, as the wheels 
 make their measured revolutions, and the valves are 
 seen gradually opening and shutting, we may have an 
 opportunity of determining the relations of the several 
 parts of the machine to one another, and of arriving 
 at just conclusions concerning the plan on which it is 
 constructed. Nor can we doubt that the parts of th rt 
 machine bear the same relation to one another, am 
 that their movements take place in precisely the same 
 order, when the supply of steam is large as \\hen it is 
 small. 
 
 Now, as we shall show in the sequel, a volcano is a 
 kind of great natural steam-engine, and our best method 
 of investigating its action is to watch it when a part of 
 the steam-supply is cut off. It is true that we cannot 
 at will control the source of supply of steam to a 
 volcano, as we can in a steam-engine, but as some 
 volcanoes have usually only a small steam-supply, and 
 nearly all volcanoes vary greatly in the intensity of 
 their action at different periods, we can, by a careful 
 selection of the object or the time of our study, gain 
 all those advantages which would be obtained by regu- 
 lating its action for ourselves. 
 
 Spallanzani appears to have been the *irst to per- 
 ceive the important fact, that the nature of volcanic 
 action remains the same, however its intensity may 
 vary. Taking advantage of the circumstance that 
 there exists in the Mediterranean Sea a volcano 
 
APPEARANCE OF STROMBOLI FROM A DISTANCE. <J 
 
 Stromboli which for at least 2,000 years has been in 
 a constant and regular, but not in a violent or dan- 
 gerous, state of activity, he visited the spot, and made 
 the series of careful observations which laid the foun- 
 dation of our knowledge of the ' physiology of vol- 
 canoes.' Since the time of Spallanzani, many other 
 investigators have visited the crater of Stromboli, and 
 they have been able to confirm and extend the ob- 
 servations of the great Italian naturalist, as to the 
 character of the action which is constantly taking place 
 within it. We cannot better illustrate the nature of 
 volcanic action than by describing what has been wit- 
 nessed by numerous observers within the crater of 
 Stromboli, where it is possible to watch the series of 
 operations going on by the hour together, and to do so 
 without having our judgment warped either by an 
 excited imagination or the sense of danger. 
 
 In the sketch, fig. 1, which was made on April 
 20, 1874, I have shown the appearance which this 
 interesting volcano usually presents, when viewed from 
 a distance. The island is of rudely circular outline, 
 and conical form, and rises to the height of 3,090 feet 
 above the level of the Mediterranean. From a point 
 on the side of the mountain, masses of vapour are 
 seen to issue, and these unite to form a cloud over the 
 mountain, the outline of this vapour-cloud varying 
 continually according to the hygrometric state of the 
 atmosphere, and the direction and force of the wind. 
 At the time when this sketch was made, the vapour- 
 2 
 
10 VOLCANOES. 
 
 cloud was spread in a great horizontal stratum over- 
 shadowing the whole island, but it was clearly seen to 
 be made up of a number of globular masses, each of 
 which, as we shall hereafter see, is the product of a 
 distinct outburst of the volcanic forces. 
 
 Viewed at night-time, Stromboli presents a far 
 more striking and singular spectacle. The mountain, 
 owing to its great elevation, is visible over an area 
 having a radius of more than 100 miles. When 
 watched from the deck of a vessel anywhere within 
 this area, a glow of red light is seen to make its ap- 
 pearance from time to time above the summit of the 
 mountain ; this glow of light may be observed to 
 increase gradually in intensity, and then as gradually 
 to die away. After a short interval the same appear- 
 ances are repeated, and this goes on till the increasing 
 light of the dawn causes the phenomenon to be no 
 longer visible. The resemblance presented by Strom- 
 boli to a ' flashing light ' on a most gigantic scale is 
 very striking, and the mountain has long been known 
 as i the lighthouse of the Mediterranean.' 
 
 It must be pointed out, however, that in two very 
 important particulars the appearances presented by 
 Strornboli differ markedly from those rhythmical gleams 
 exhibited by the 6 flashing-lights ' of our coasts. In 
 the first place, the intervals between successive flashes 
 are very unequal, varying from less than one minute 
 to twenty minutes, or even more ; and in the second 
 place, the duration and intensity of the red glow above 
 
GENERAL FEATURES OF THE MOUNTAIN. 
 
 11 
 
 the mountain are subject to like variation, being some- 
 times a momentary scarcely visible gleam, and at others 
 a vivid burst of light which illuminates the sky to a 
 considerable distance round. 
 
 Let us now draw near and examine this wonderful 
 phenomenon of a mountain which seemingly ever 
 
 FIG. 2. MAP OF THE ISLAND OF STROMBOLI 
 (Scale about two inches to a mile.) 
 
 burns with fire, and yet is not consumed. The general 
 form of the Island of Stromboli w r ill be gathered from 
 an inspection of the plan, fig. 2, which is copied from 
 a map published by the Italian Government. When 
 we land upon the island, we find that it is entirely 
 built up of such materials as we know to be ejected 
 
12 VOLCANOES. 
 
 from volcanoes; indeed, it resembles on a gigantic 
 scale the surroundings of an iron furnace, with its 
 heaps of cinders and masses of slag. The irregularity 
 in the form of the island is at once seen to be due to 
 the action of the wind, the rain, and the waves of the 
 surrounding sea, which have removed the loose, cin- 
 dery materials at some points, and left the hard, slaggy 
 masses standing up prominently at others. 
 
 This great heap of cindery and slaggy materials 
 rises, as we have said, to a height of more than 3,000 
 feet above the sea-level, but even this measurement 
 does not give a just idea of its vast bulk. Soundings 
 in the sea surrounding the island show that the 
 bottom gradually shelves around the shores to the 
 depth of nearly 600 fathoms, so that Stromboli is a 
 great conical mass of cinders and slaggy materials, 
 having a height of over 6,000 feet, and a base whose 
 diameter exceeds four miles. 
 
 The general form and proportions of this mass will 
 be better understood by an examination of the section, 
 fig. 3, which is also constructed from the materials 
 furnished by ihe map of the island issued by the 
 Italian Government. The same section, and the map, 
 fig. 2, will serve to make clear the position and re- 
 lations of the point on the mountain at which the 
 volcanic activity takes place. At a spot on the north- 
 west slope of the mountain, about 1,000 feet below its 
 summit, and 2,000 feet above the level of the sea, 
 there exists a circular depression, the present active 
 
FORM AND POSITION OF THE CRATER. 13 
 
 6 crater ' of the volcano ; and leading down from this 
 to the sea there is a flat slope making an angle of 
 about 35 with the horizon, and known as the ' Sci- 
 arra.' The Sciarra is bounded by steep cliffs, as shown 
 in the sketch fig. 1, and the plan fig. 2. 
 
 If w r e climb up to this scene of volcanic activity, 
 we shall be able to watch narrowly the operations 
 which are going on there. On the morning of the 
 
 FIG. 3. SECTION THROUGH THE ISLAND OF STROMBOLI FROM N.W. TO s E. 
 
 . Highest summit of the mountain, c. Cratere del Fossa. 6. Point overlooking 
 the crater, d. Steep slope known as the Sciarra del Fuoco. e. Continua- 
 tion of the same slope beneath the level of the sea. /. Steep cliffs of the 
 Punta dell' Omo. 
 
 24th of April, 1874, I paid a visit to this interesting 
 spot in order to get a near view of what was taking 
 place. On reaching a point upon the side of the 
 Sciarra, from which the crater was in full view before 
 me, I witnessed, and made a sketch of, an outburst 
 which then took place, and this sketch has been re- 
 produced in fig. 4. Before the outburst, numerous 
 light curling wreaths of vapour were seen ascending 
 from fissures on the sides and bottom of the crater. 
 Suddenly, and without the slightest warning, a sound 
 was heard like that produced when a locomotive blows 
 
14 
 
 VOLCANOES. 
 
 off its steam at a railway-station ; a great volume of 
 watery vapour was at the same time thrown violently 
 into the atmosphere, and with it there were hurled 
 upwards a number of dark fragments, which rose to 
 the height of 400 or 500 feet above the crater, de- 
 scribing curves in their course, and then falling 
 back upon the mountain. Most of these fragments 
 
 FIG. 4. THE CRATER OF STROMBOLI AS VIEWED FROM THE SIDE OF THE 
 SCIARRA DURING AN ERUPTION ON THE MORNING OF APRIL 24, 1874. 
 
 tumbled into the crater with a loud, rattling noise, but 
 some of them fell outside the crater, and a few rolled 
 down the steep slope of the Sciarra into the sea. 
 Some of these falling fragments were found to be 
 still hot and glowing, and in a semi-molten condition, 
 so that they readily received the impression of a coin 
 thrust into them. 
 
 But on the upper side of the crater, at the point 
 
APERTURES AT THE BOTTOM OF THE CRATER, 15 
 
 marked 6, on the section fig. 3, there exists a spot 
 which we can look down upon the bottom of the crater, 
 and view the operations taking place there. This is 
 the place where Spallanzani and other later investigators 
 have carried on their observations, and, when the wind 
 is blowing from the spectator towards the crater, he 
 may sit for hours watching the wonderful scene dis- 
 played before him. The black slaggy bottom of the 
 crater is seen to be traversed by many fissures or 
 cracks, from most of which curling jets of vapour issue 
 quietly, and gradually mingle with and disappear in 
 the atmosphere. But besides these smaller cracks at 
 the bottom of the crater, several larger openings are 
 seen, which vary in number and position at different 
 periods ; sometimes only one of these apertures is 
 visible, at others as many as six or seven, and the 
 phenomena presented at these larger apertures are 
 especially worthy of careful investigation. 
 
 These larger apertures, if we study the nature of 
 the action taking place at them, may be divided into 
 three classes. From those of the first class, steam is 
 emitted with loud, snorting puffs, like those produced by 
 a locomotive-engine, but far less regular and rhythmical 
 in their succession. In the second class of apertures 
 masses of molten material are seen welling out, and, if 
 the position of the aperture be favourable, flowing out- 
 side the crater; from this liquid molten mass steam 
 is seen to escape, sometimes in considerable quantities. 
 The openings of the third class present still more 
 
i6 VOLCANOES. 
 
 interesting appearances. Within the walls of the 
 aperture a viscid or semi-liquid substance is seen slowly 
 heaving up and down. As we watch the seething mass 
 the agitation within it is observed to increase gradually, 
 and at last a gigantic bubble is formed which violently 
 bursts, when a great rush of steam takes place, carry- 
 ing fragments of the scum-like surface of the liquid 
 high into the atmosphere. 
 
 If we visit the crater by night, the appearances 
 presented are found to be still more striking and 
 suggestive. The smaller cracks and larger openings 
 glow with a ruddy light. The liquid matter is seen to 
 be red- or even white-hot, while the scum or crust which 
 forms upon it is of a dull red colour. Every time a 
 bubble bursts and the crust is broken up by the escape 
 of steam, a fresh, glowing surface of the incandescent 
 material is exposed. If at these moments we look up 
 at the vapour-cloud covering the mountain, we shall at 
 once understand the cause of the singular appearances 
 presented by Stromboli when viewed from a distance 
 at night, for the great masses of vapour are seen to be 
 lit up with a vivid, ruddy glow, like that produced when 
 an engine-driver opens the door of the furnace and 
 illuminates the stream of vapour issuing from the 
 funnel of his locomotive. 
 
 Let us now endeavour to analyse the phenomena so 
 admirably displayed before us in the crater of Stromboli. 
 The three essential conditions on which the production 
 of these phenomena seems to depend are the following: 
 
CAUSE OF THE GLOWING- LIGHT. 17 
 
 first, the existence of certain apertures or cracks com- 
 municating between the interior and the surface of the 
 earth ; secondly, the presence of matter in a highly 
 heated condition beneath the surface ; and thirdly, the 
 existence of great quantities of water imprisoned in the 
 subterranean regions which water, escaping as steam, 
 gives rise to all those active phenomena we have been 
 describing. 
 
 We have said, at the outset, that there exists no 
 analogy whatever between the action which takes place 
 in volcanoes and the operation of burning or combustion. 
 Occasionally, it is true, certain inflammable substances 
 are formed by the action going on within the volcano, 
 and these inflammable substances, taking fire, produce 
 real flames. Such flames are, however, in almost all 
 cases only feebly luminous, and do not give rise to any 
 conspicuous appearances. What is usually taken for 
 flame during volcanic eruptions is simply, as we have 
 already pointed out, the glowing red-hot surface of a 
 mass of molten rock, reflected from a vapour-cloud 
 hanging over it. The red glow observed over a volcano 
 in eruption is indeed precisely similar in its nature and 
 origin to that which is seen above London during a 
 night of heavy fog, and which is produced by the re- 
 flection of the gas-lights of the city from the innumer- 
 able particles of water-vapour diffused through the 
 atmosphere. Fires, of course, occur when the molten 
 and incandescent materials poured out from a volcano 
 come in contact with inflammable substances, such as 
 
18 VOLCANOES. 
 
 forests and houses, but in these cases the combustion 
 is quite a secondary phenomenon. 
 
 There is another popular delusion concerning vol- 
 canic action, which it may be necessary to refer to and 
 to combat. From the well-known fact that sulphur or 
 brimstone is found abundantly in volcanic regions, the 
 popular belief has arisen that this highly inflammable 
 substance has something to do with the production of 
 the eruptions of volcanoes. In school-books which were, 
 until comparatively recent years, in constant use in this 
 country, the statement may be found that by burying 
 certain quantities of sulphur, iron -pyrites, and charcoal 
 in a hole in the ground, we may form a miniature 
 volcano, and produce all the essential phenomena of a 
 volcanic eruption. No greater mistake could possibly 
 be made. The chemical reactions which take place 
 when sulphur and other substances are made to act 
 upon each other differ entirely from the phenomena 
 of volcanic action. The sulphur which is found in 
 volcanic regions is the result and not the cause of 
 volcanic action. Among the most common substances 
 emitted from volcanic vents along with the steam are 
 the two gases, sulphurous acid and sulphuretted hy- 
 drogen. When these two gases come into contact 
 with one another, chemical action takes place, and the 
 elements contained in them oxygen, hydrogen, and 
 sulphur are free to group themselves together in an 
 entirely new fashion ; the consequence of this is that 
 water and sulphuric acid (oil of vitriol) are formed, and 
 
VOLCANIC ACTION EKSEMBLES BOILING. 19 
 
 a certain quantity of sulphur is set free. The water 
 escapes into the atmosphere, the sulphuric acid 'com- 
 bines with lime, iron, or other substances contained in 
 the surrounding rocks, and the sulphur builds up crys- 
 tals in any cavities which may happen to exist in these 
 rocks. 
 
 If, however, careful and exact observations, like 
 those carried on at Stromboli, compel us to reject the 
 popular notions concerning the supposed resemblance 
 between volcanic action and the combustion of sulphur 
 or other substances, they nevertheless suggest analo- 
 gies with certain other simple and well-known opera- 
 tions. And in pursuing these analogies, we are led to 
 the recognition of some admirable illustrations both 
 of the attendant phenomena and of the true cause of 
 volcanic outbursts. 
 
 No one can look down on the mass of seething 
 material in violent agitation within the fissures at the 
 bottom of the crater of Stromboli, without being forcibly 
 reminded of the appearances presented by liquids in 
 a state of boiling or ebullition. The glowing material 
 seems to be agitated by two kinds of movements, the 
 one whirling or rotatory, the other vertical or up-and- 
 down in its direction. The fluid mass in this way 
 appears to be gradually impelled upwards, till it ap- 
 proaches the lips of the aperture, when vast bubbles 
 are formed upon its surface, and to the sudden burst- 
 ing of these the phenomena of the eruption are due. 
 
 Now if we take a tall narrow vessel and fill it with 
 
20 VOLCANOES. 
 
 porridge or some similar substance of imperfect fluidity, 
 we shall be able, by placing it over a fire, to imitate 
 very closely indeed the appearances presented in the 
 crater of Stromboli. As the temperature of the mass 
 rises, steam is generated within it, and in the efforts of 
 this steam to escape, the substance is set in violent 
 movement. These movements of the mass are partly 
 rotatory and partly vertical in their direction ; as fresh 
 steam is generated in the mass its surface is gradually 
 raised, while an escape of the steam is immediately 
 followed by a fall of the surface. Thus an up-and- 
 down movement of the liquid is maintained, but as the 
 generation of steam goes on faster than it can escape 
 through the viscid mass, there is a constant tendency 
 in the latter to rise towards the mouth of the vessel. 
 At last, as we know, if heat continues to be applied to 
 the vessel, the fluid contents will be forced up to its 
 edge and a catastrophe will occur; the steam being 
 suddenly and violently liberated from the bubbles 
 formed on the surface of the mass, and a considerable 
 quantity of the material forcibly expelled from the 
 vessel. The suddenness and violence of this cata- 
 strophe is easily accounted for, if we bear in mind that 
 the escaping steam acts after the manner of a com- 
 pressed spring which is suddenly released. Steam is 
 first formed at the bottom of the vessel which is in 
 contact with the fire ; but here it is under the pressure 
 of the whole mass of the liquid, and moreover, the 
 viscidity of the substance tends to retard the union of 
 
ESCAPE OF STEAM-BUBBLES FEOM LAVA. 21 
 
 the steam bubbles and their rise to the surface of the 
 mass. But when the pressure is relieved by the bur&t 
 ing of bubbles at the surface, the whole of the generated 
 steam tends to escape suddenly. 
 
 Now within the crater of Stromboli we have pre- 
 cisely the necessary conditions for the display of the 
 same series of operations. In the apertures at the 
 bottom there exists a quantity of imperfectly fluid 
 materials at a higher temperature, containing water 
 entangled in its mass. As this water passes into the 
 state of steam it tends to escape, and in so doing puts 
 the whole mass into violent movement. When the steam 
 rises to the surface, bubbles are formed, and the forma- 
 tion of these bubbles is promoted by the circumstance 
 that the liquid mass, where exposed to the atmosphere, 
 becomes chilled, and thereby rendered less perfectly 
 fluid. By the bursting of these bubbles the pressure 
 is partially relieved, and a violent escape of the pent- 
 up steam takes place through the whole mass. Equili- 
 brium being thus restored, there follows a longer or 
 shorter interval of quiescence, during which steam is 
 being generated and collected within the mass, and 
 the series of operations which we have described then 
 recommences. 
 
 There is one other consideration which must be 
 borne in mind in connection with this subject. It is 
 well known that if water be subjected to sufficiently 
 great pressure it may be raised to a very high tempe- 
 rature and still retain its liquid condition. When this 
 
22 VOLCANOES. 
 
 pressure is removed, however, the whole mass passes 
 at once into the condition of steam or water-gas ; and 
 the gas thus formed at high temperatures has a pro- 
 portionably high tension. In a Papin's digester water 
 confined in a strong vessel is raised to temperatures 
 far above its ordinary boiling-point, and from any 
 opening in such a vessel the steam escapes with pro- 
 digious violence. Now, at considerable depths beneath 
 the earth's surface, and under the pressure of many 
 hundreds or thousands of feet of solid rock, water still 
 retaining its liquid condition may become intensely 
 heated. When the pressure is relieved by the forma- 
 tion of a crack or fissure in the superincumbent mass 
 of rock, the escape of the superheated steam will be 
 of very violent character, and may be attended with 
 the most striking and destructive results. In the 
 existence of high temperatures beneath the earth's 
 surface, and the presence in the same regions of 
 imprisoned water capable of passing into the highly 
 elastic gas which we call steam, we have a cause fully 
 competent to produce all the phenomena which we 
 have described as" occurring at Stromboli. 
 
 It may at first sight appear that the grand and 
 terrible displays of violence witnessed during a great 
 volcanic eruption differ fundamentally in their cha- 
 racter and their origin from those feeble outbursts 
 which we are able to examine closely and analyse 
 rigorously at Stromboli. But that such is not the case 
 a few simple considerations will soon convince us. 
 
STKOMBOLI COMPAEED WITH VESUVIUS. 23 
 
 Although Stromboli usually displays the subdued 
 and moderate activity which we have been describing, 
 yet the intensity of the action going on within it is 
 subject to considerable variation. Occasionally the 
 violence of the outbursts is greatly increased the 
 roaring of the steam-jets may be heard for many miles 
 around, considerable streams of incandescent liquefied 
 rock flow down the Sciarra into the sea, and the ex- 
 plosions in the crater are far more frequent and ener- 
 getic, cinders and fragments of rock being scattered 
 all over the island and the surrounding seas. 
 
 On the other hand, volcanoes like Vesuvius, which 
 are sometimes the scene of eruptions on the very 
 grandest scale, at others subside into a temporary state 
 of moderate activity quite similar in character to that 
 which is the normal condition of Stromboli. Thus, 
 shortly before the great eruption of Vesuvius in April 
 1872, a small cone was formed near the edge of the 
 crater, and during some months observers could watch, 
 in ease and safety, a series of small explosions taking 
 place, quite similar in their character and attendant 
 phenomena to those which we have described as oc- 
 curring at Stromboli. French geologists are in the 
 habit of defining the condition of activity in a volcano 
 by speaking of the more quiet and regular state as the 
 ' Strombolian stage,' and the more violent and paroxys- 
 mal as the ' Vesuvian stage ' ; but the two conditions 
 are, as we have seen, presented by the same volcano at 
 different periods, and pass into one another by the 
 most insensible gradations. 
 
24 VOLCANOES. 
 
 We must now proceed to compare the grand and 
 terrible appearances presented during a great eruption 
 with those more feeble displays which we have been 
 describing, to show that in all their essential features 
 these different kinds of outbursts are identical with 
 one another, and must be referred to the action of 
 similar causes. 
 
 The volcanic eruption which has been most care- 
 fully studied in recent times is that which we have 
 already referred to as occurring at Vesuvius, in the 
 month of April 1872. With the exception, perhaps, 
 of that which took place in October 1822, this eruption 
 was the grandest which has broken out at Vesuvius 
 during the present century. Owing to the circum- 
 stance of its proximity to the great city of Naples, 
 Vesuvius has always been the most carefully watched 
 of all volcanoes, and in recent years the erection of an 
 observatory, provided with instruments for recording 
 the smallest subterranean tremors affecting the moun- 
 tain, has facilitated the carrying on of those continuous 
 and minute observations which are so necessary for 
 exact scientific inquiry. 
 
 On the occasion of this outburst, the aid of in- 
 stantaneous photography was first made available for 
 obtaining a permanent record of the appearances dis- 
 played at volcanic eruptions. In fig. 5 we have one of 
 these photographs, which was taken at 5 o'clock P.M. 
 on April 26, 1872, transferred to a wood-block and 
 engraved. In examining it we feel sure that we 
 
VESUVIUS ERUPTION OF 1872. 25 
 
 are not being misled by any exaggeration or error on 
 the part of the artist. Vesuvius rises to the height 
 of nearly 4,000 feet above the level of the sea, and an 
 inspection of the photograph proves that the vapours 
 and rock-fragments were thrown to the enormous 
 height of 20,000 feet, or nearly four miles, into the 
 atmosphere. 
 
 The main features of this terrifying outburst were 
 as follows. For more than a twelvemonth before, the 
 activity of the forces at work within the mountain 
 appeared to be gradually increasing, and the great 
 eruption commenced on April 24, attained its climax 
 on the 26th, and began to die out on the following 
 day. During the eruption the bottom of the crater 
 was entirely broken up, and the sides of the mountain 
 were rent by fissures in all directions. So numerous 
 were these fissures and cracks that liquid matter 
 appeared to be oozing from every part of its surface, 
 and, as Professor Palmieri, who witnessed the outburst 
 from the observatory, expressed it, 'Vesuvius sweated 
 fire.' One of the fissures was of enormous size, ex- 
 tending from the summit to far beyond the base of the 
 cone ; the scar left by this gigantic rent being plainly 
 visible at the present day. 
 
 From the great opening or crater at the summit, 
 and from some of the fissures on the sides of the 
 mountain, enormous volumes of steam rushed out with 
 a prodigious roaring sound, the noise being so terrific 
 that the inhabitants of Naples, five miles off, fled from 
 
26 VOLCANOES. 
 
 their houses and spent the night in the open streets. 
 Although this roaring sound appeared at a distance to 
 be continuous, yet those upon the mountain could per- 
 ceive that it was produced by detonations or explosions 
 rapidly following one another. Each of these ex- 
 plosions was accompanied by the formation of a great 
 globe of white vapour, which, rising into the atmo- 
 sphere, swelled the bulk of the vast cloud overhanging 
 the mountain. An inspection of the photographs (see 
 fig. 5) shows that the great vapour-cloud over Vesuvius 
 was made up of the globular masses ejected at succes- 
 sive explosions. Each of these explosive upward rushes 
 of steam carried along with it a considerable quantity 
 of solid fragments, and these fell in great numbers all 
 over the surface of the mountain, breaking the windows 
 of the observatory, and making it dangerous to be out 
 of doors. 
 
 We have said that lava, or molten rock, appeared 
 to be issuing from the very numerous cracks formed 
 all over the flanks of the mountain. But at three 
 points this molten rock issued in such quantities as to 
 form great, fiery floods, which rushed down the sides 
 of the mountain, and flowed to a considerable distance 
 beyond its base. The largest of these lava-floods over- 
 whelmed and destroyed the two villages of Massa di 
 Somma and San Sebastiano, besides many country 
 houses in the neighbourhood. 
 
 A very marked and interesting feature exhibited 
 by these three lava-floods was the quantity of watery 
 
STEAM EMITIED FROM LAVA-CURRENTS. 27 
 
 vapour which they gave off during their flow. All 
 along their course, enormous volumes of steam were 
 evolved from them, as will be seen by an inspection of 
 the photograph. Indeed, such was the abundance and 
 tension of the steam thus escaping from the surfaces 
 of the lava-currents that it forced the congealing rock 
 up into great bubbles and blisters, and gave rise to the 
 formation of innumerable miniature volcanoes, varying 
 in size from a beehive to a cottage, some of which 
 remained in a state of independent activity for a con- 
 siderable time. 
 
 So far, what we have described as taking place at 
 Vesuvius, in April 1872, has been only the repetition 
 on a far grander scale of the three kinds of action 
 which we have shown to be constantly taking place at 
 Stromboli ; namely, the formation of cracks or fissures 
 in the earth's surface, the escape of steam with ex- 
 plosive violence from these openings, often propelling 
 rock-fragments into the atmosphere, and the out-well- 
 ing, under the influence of this compressed steam, of 
 masses of molten materials. 
 
 There were some other appearances presented at 
 the great outburst at Vesuvius, which do not seem at 
 first sight to find any analogies in the manifestations 
 of the more feeble action continually going on at 
 Stromboli. 
 
 Before and during the great outbreak of April 1872, 
 Vesuvius itself and the whole country round were 
 visited with earthquake-shocks, or tremblings of the 
 
28 VOLCANOES. 
 
 ground. The sensitive instruments in the Veeuvian 
 Observatory showed the mountain during the eruption 
 to be in a constant state of tremor. These earthquakes 
 are not, as is commonly supposed, actual upheavings 
 of the earth's surface, but are vibrations propagated 
 through the solid materials of which the earth is built 
 up. We cannot stamp our feet upon the ground 
 without giving rise to such vibrations, though our 
 senses may not be sufficiently acute to perceive them. 
 The explosive escape of steam from a crack is a cause 
 sufficiently powerful to produce a shock which is pro- 
 pagated and may be felt for a considerable distance 
 round. Even on Stromboli an observer at the edge of 
 the crater may notice that each explosive outburst of 
 steam is accompanied by a perceptible tremor of the 
 ground, and in the case of Vesuvius the violent shocks 
 produced by the escape of far larger volumes of steam 
 give rise to proportionately stronger vibrations. The 
 nature and origin of those far more terrible and de- 
 structive shocks which sometimes accompany, and 
 more frequently precede, great volcanic eruptions, we 
 shall consider in the sequel. 
 
 Another striking phenomenon which was exhibited 
 in the great eruption of Vesuvius in 1872 was the 
 vivid display of lightning accompanied by thunder. 
 The uprushing current of steam and rock-fragments 
 forms a vertical column, but as the steam condenses it 
 spreads out into a great horizontal cloud which is seen 
 to be made up of the great globes of vapour emitted at 
 
CAUSE OF LIGHTNING DUKING ERUPTIONS. 29 
 
 successive explosions. When there is little or no wind 
 the vertical column with a horizontal cloud above it 
 bears a striking resemblance to the stone-pine trees 
 which form so conspicuous a feature in every Neapolitan 
 landscape. Around this column of vapour the most 
 vivid lightning constantly plays and adds not a little 
 to the grand and awful character of the spectacle of a 
 volcanic eruption, especially when it is viewed by night. 
 
 In the eruption of 1872 a strong wind blowing 
 from the north-west destroyed the usual regular ap- 
 pearance of this ' pine-tree appendage ' to the moun- 
 tain, which is so well known to, and dreaded by the in- 
 habitants of Naples; the cloud, as will be seen from 
 the photograph (fig. 5. facing p. 24), was blown on one 
 side, and most of the falling fragments took the same 
 direction. 
 
 It is well known that when high-pressure steam 
 is allowed to escape through an orifice, electricity is 
 abundantly generated by the friction, and Sir William 
 Armstrong's hydro-electric machine is constructed on 
 this principle. Every volcano in violent eruption is a 
 very efficient hydro-electric machine, and the up- 
 rushing column is in a condition of intense electrical 
 excitation. This result is probably aided by the friction 
 of the solid particles as they are propelled upwards and 
 fall back into the crater. The restoration of the con- 
 dition of electrical stability between this column and 
 the surrounding atmosphere is attended with the pro- 
 duction of frequent lightning-flashes and thunder-claps, 
 
30 VOLCANOES. 
 
 the sound of the latter being usually, however, drowned 
 in the still louder roar of the uprushing steam-column. 
 
 The discharge of such large quantities of steam into 
 the atmosphere soon causes the latter to be saturated 
 with watery vapour, and there follows an excessive rain- 
 fall ; long-continued rain and floods were an accompani- 
 ment of the great Vesuvian outbreak of 1872, as they 
 have been of almost all great volcanic eruptions. The 
 Italians, indeed, dread the floods which follow an erup- 
 tion more than the fiery streams of lava which accom- 
 pany it for they have found the mud-streams (lave 
 difango), formed by rain-water sweeping along the loose 
 volcanic materials, to be more widely destructive in their 
 effects than the currents of molten rock (lave difuoco). 
 
 Besides the phenomena which we have now de- 
 scribed as accompanying a great volcanic outburst, 
 many others have undoubtedly been recorded by ap- 
 parently trustworthy authorities. But, in dealing with 
 the descriptions of these grand and terrible events, we 
 must always be on our guard against accepting as 
 literal facts, the statements made by witnesses, often 
 writing at some distance from the scene of action, and 
 almost always under the influence of violent excitement 
 and terror. The desire to administer to the universal 
 love of the marvellous, and the tendency to exaggera- 
 tion, will usually account for many of the wonderful 
 statements contained in such records ; and, even where 
 the witness is accurately relating events which he thinks 
 passed before his eyes, we must remember that it i? 
 
SIMILARITY OF FEEBLE AND VIOLENT ERUPTIONS. 31 
 
 probable he may have had neither the opportunity nor 
 the capacity for exact observation. 
 
 The more carefully we sift the accounts which have 
 been preserved of great volcanic outbursts, the more 
 are we struck by the fact that the appearances described 
 can be resolved into a few simple operations, the true 
 character of which has been distorted or disguised by 
 the want of accurate observation on the part of the 
 witnesses. 
 
 We are thus led to the conclusion that the grand 
 and terrible appearances displayed at Vesuvius and 
 other volcanoes in a state of violent eruption do not 
 differ in any essential respect from the phenomena 
 which we have witnessed accompanying the miniature 
 outbursts of Stromboli. And we are convinced, by 
 the same considerations, that the forces which give 
 rise to the feeble displays in the latter case would pro- 
 duce, if acting with greater intensity and violence, all 
 the magnificent spectacles presented in the former. 
 
 In Vesuvius and Stromboli alike, the active cause 
 of all the phenomena exhibited is found to be the 
 escape of steam from the midst of masses of incande- 
 scent liquefied rock. The violence, and therefore the 
 grandeur and destructive effects of an eruption, depend 
 upon the abundance and tension of this escaping steam. 
 
 There is one respect in which volcanic phenomena 
 are especially calculated to excite the fear and wonder 
 of beholders namely, in the sudden and apparently 
 spontaneous character of their manifestations. Eclipses 
 
32 VOLCANOES 
 
 were regarded as equally portentous with volcanic erup- 
 tions till astronomers learned not only to explain the 
 causes which gave rise to them, but even to predict to 
 the second the times of their occurrence. If we were 
 able in like manner to warn the inhabitants of volcanic 
 regions of the approach of a grand eruption, the fear 
 and superstition with which these events are now re- 
 garded would doubtless be in great part dispelled. 
 The power of prediction is alike the crucial test and 
 the crowning triumph of a scientific theory. 
 
 But, although natural philosophers are able to assign 
 the causes to which the grand operations of volcanoes 
 are due, and also to explain all the varied appearances 
 which accompany them, they have not as yet so far 
 mastered the laws which govern volcanic action as to 
 be able to predict the periods of their manifestation. 
 
 That these operations, like all others going on upon 
 the globe, are governed by great natural laws we cannot 
 for a moment doubt. And that, in all probability, 
 more careful and exact observation and reasoning will 
 at some future time lead us to the recognition of 
 these laws, every student of nature is sanguine. But 
 at the present time, it must be confessed, we are very 
 far indeed from being able to afford that crowning 
 proof of the truth, of our theories of volcanic action 
 which is implied in the power of predicting the period 
 and degree of intensity of their manifestations. 
 
 There are, however, some observations which lead 
 ns to hope that the time may not be far distant when 
 
ERUPTIONS AND THE INTERVALS BETWEEN THEM. 33 
 
 we shall have so far obtained a knowledge of the con- 
 ditions on which volcanic action depends as to be able 
 to form some judgment as to its manifestations in the 
 future at any particular locality. But we must recol- 
 lect that these conditions are very numerous and com- 
 plicated, and that some of them may lie almost entirely 
 outside our sphere of observation ; hence hasty attempts 
 in this direction, such as have recently been made, are 
 to be deprecated by every true lover of science. 
 
 Concerning the eruptions that have taken place at 
 those volcanic centres which have been known from a 
 remote antiquity, we have records from which we can 
 determine the intervals separating these outbursts and 
 their relative violence. A critical examination of these 
 records leads to the following conclusions : 
 
 (1.) A long period of quiescence is generally fol- 
 lowed by an eruption which is either of long duration 
 or of great violence. 
 
 (2.) A long-continued, or very violent eruption is 
 usually followed by a prolonged period of repose. 
 
 (3.) Feeble and short eruptions usually succeed 
 one another at brief intervals. 
 
 (4.) As a general rule, the violence of a great erup- 
 tion is inversely proportional to its duration. 
 
 It will be seen that these general conclusions are 
 in perfect harmony with the theory that volcanic out- 
 bursts are due to the accumulation of steam at vol- 
 canic centres, and that the tension of this imprisoned 
 gas eventually overcomes the repressing forces which 
 3 
 
34 VOLCANOES. 
 
 tend to prevent its manifestation. Before astronomers 
 had learnt to determine all the conditions on which 
 the production of eclipses depends, they had found 
 that these phenomena succeed one another at regular 
 intervals. The discovery of such astronomical cycles 
 was a great advance in our knowledge of the heavenly 
 bodies, and in the same way the determination of 
 these general relations between the intensity and 
 duration of volcanic outbursts and the intervals of 
 time which separate them may be regarded as the first 
 step towards the discovery of the laws which govern 
 volcanic activity. 
 
 In the actual determination of the conditions 
 upon which the occurrence of volcanic eruptions 
 depends, it must be confessed, however, that very little 
 has as yet been done. This is in part due to the fact 
 that some at least of these conditions lie beyond the 
 limits of direct observation. But it must also be ad- 
 mitted, on the other hand, that little has been as yet 
 accomplished towards the careful and systematic obser- 
 vation of those phenomena which may, and probably 
 do, exert an influence in bringing about volcanic out- 
 bursts. 
 
 In the Lipari Islands there has prevailed a belief, 
 from the very earliest period of history, that the feeble 
 eruptions of Stromboli are in some way dependent 
 upon the condition of the atmosphere. These islands 
 were known to the ancients as the ^Eolian Isles, from 
 the fact that they were once ruled over by a king of 
 
INFLUENCE OF ATMOSPHERIC CONDITIONS. 35 
 
 the name of ^Eolus. It seems not improbable that 
 ^Eolus was gifted with natural powers of observation and 
 reasoning far in advance of those of his contemporaries. 
 A careful study of the vapour-cloud which covers Strom- 
 boli would certainly afford him information concerning 
 the hygrometric condition of the atmosphere ; the form 
 and position assumed by this vapour-cloud would be a 
 no less perfect index of the direction and force of the 
 wind ; and, if the popular belief be well founded, the 
 frequence and violence of the explosions taking place 
 from the crater would indicate the barometric pressure. 
 From these data an acute observer would be able 
 to issue 'storm-warnings' and weather-prognostics of 
 considerable value. In the vulgar mind, the idea of the 
 prediction of natural events is closely bound up with 
 that of their production ; and the shrewd weather- 
 prophet of Lipari was after his death raised to the 
 rank of a god, and invested with the sovereignty of the 
 winds. 
 
 Whether the popular idea that the outbursts of 
 Stromboli are regulated by atmospheric conditions 
 has any foundation is still open to grave doubt. It 
 seems to be certain, however, that during autumn and 
 winter the more violent paroxysms of the volcano 
 occur, and that in summer the action which takes 
 place is far more regular and equable. It would be of 
 the greatest benefit to science if an observatory were 
 erected beside the crater of Stromboli, where a care- 
 ful record might be kept of all atmospheric changes, 
 
36 VOLCANOES. 
 
 and of the synchronous manifestations of the volcanic 
 forces. 
 
 A little consideration will show that it is a by no 
 means unreasonable supposition that variations in at- 
 mospheric pressure 'may exercise a very important in- 
 fluence in bringing about volcanic outbursts. Changes 
 in the barometer to the extent of two inches within a 
 very short period are not uncommon* occurrences. A 
 very simple calculation will show that the fall of the 
 mercury in the barometer to the extent of two inches 
 indicates the removal of a weight of two millions of 
 tons from each square mile of the earth's surface where 
 this change takes place. Now, if we suppose, as we 
 have good ground for doing, that under volcanic areas 
 vast quantities of superheated water are only prevented 
 from flashing into steam by the superincumbent pres- 
 sure, a relief of this pressure to the extent of two 
 millions of tons on every square mile could scarcely 
 fail to produce very marked effects. The way in which 
 explosions in fiery coal-mines generally follow closely 
 upon sudden falls in the atmospheric pressure is now 
 well known ; and coal-mine explosions and volcanic 
 outbursts have this in common, that both result from 
 the sudden and violent liberation of subterranean 
 gases. There are not a few apparently well-authenti- 
 cated accounts of volcanic and earthquake phenomena 
 following closely on peculiar atmospheric conditions, 
 and the whole question of the relation of the volcanic 
 forces to atmospheric pressure, as Spallanzani himself 
 
SUPPOSED TIDAL EFFECTS. 37 
 
 so long ago pointed out, is deserving of a most careful 
 and rigorous investigation. 
 
 There is one other consideration which has fre- 
 quently been urged as worthy of especial attention, in 
 dealing with the question of the exciting causes of 
 volcanic outbursts. If volcanoes were, as was at one 
 time almost universally supposed, in direct communi- 
 cation with a great central mass of liquefied materials, 
 or even if any large reservoirs of such liquids existed 
 beneath volcanic districts, as others have imagined, 
 then the different mobility of the solid and liquid por- 
 tions of the earth's mass would give rise to tidal effects 
 similar to those occurring in the surface waters of the 
 globe. Under such circumstances, volcanic outbursts, 
 like the tides, would be determined by the relative 
 positions of the sun and moon to our globe. It is cer- 
 tain, however, that no very direct relation has yet been 
 established between the lunar periods and those of 
 volcanic outbursts, though recent close observations 
 upon the crater of Vesuvius, by Professor Palmieri, do 
 seem to lend support to the view that such relations 
 may exist. 
 
 At the present time, therefore, it must be admitted 
 that vulcanologists have only just commenced those 
 series of exact and continuous observations which are 
 necessary to determine the conditions that regulate the 
 appearance of volcanic phenomena. The study of the 
 laws of volcanic action is yet in its infancy. But the 
 establishment of observatories on Vesuvius and Etna 
 
38 VOLCANOES. 
 
 is full of promise for the future, and when we con- 
 sider the advances which have been made, during the 
 last one hundred years, in our knowledge of the true 
 nature of volcanic action, we need not despair that the 
 extension of the same methods of inquiry will lead to 
 equally important results concerning the conditions 
 which determine and the laws which govern it. 
 
 In the meanwhile, it is no small gain to have estab- 
 lished the fact that volcanic phenomena, divested of 
 all those wonderful attributes with which superstition 
 and the love of the marvellous have surrounded them, 
 are operations of nature obeying definite laws, which 
 laws we may hope by careful observation and accurate 
 reasoning to determine ; and that the varied appear- 
 ances, presented alike in the grandest and feeblest 
 outbursts, can all be referred to one simple cause 
 namely, the escape, from the midst of masses of molten 
 materials, of imprisoned steam or water-gas. 
 
CHAPTEK III. 
 
 THE PRODUCTS OF VOLCANIC ACTION. 
 
 WHILE SpalLmzani was engaged in investigating the 
 nature of the action going on at Stromboli and other 
 Italian volcanoes, his contemporary Dolomieu was laying 
 the foundation of another important branch of vulcan- 
 ology by studying the characters of the different ma- 
 terials of which volcanoes are built up. Since the pub- 
 lication of Dolomieu's admirable works on the rocks of 
 the Lipari and Ponza Islands, science has advanced with 
 prodigious strides. The chemist has taught us how 
 to split up a rock into its constituent elements and 
 to determine the proportions of these to one another 
 with mathematical precision ; the mineralogist has 
 done much in the investigation of the characters and 
 mode of origin of the crystalline minerals which occur 
 in these rocks ; and the microscopist has shown how the 
 minute internal structure of these rocks may be made 
 clearly manifest. We shall proceed to give a sketch of 
 the present state of knowledge obtained by these dif- 
 ferent kinds of investigations, concerning the materials 
 which are ejected from volcanic vents. 
 
40 VOLCANOES 
 
 The most abundant of the substances which are 
 ejected from volcanoes is steam or water-gas, which, as 
 we have seen, issues in prodigious quantities during 
 every eruption But with the steam a great number of 
 other volatile materials frequently make their appear- 
 ance. The chief among these are the acid gases known 
 as hydrochloric acid, sulphurous acid, sulphuretted 
 hydrogen, carbonic acid, and boracic acid; and with 
 these acid gases there issue hydrogen, nitrogen, am- 
 monia, the volatile metals arsenic, antimony, and mer- 
 cury, and some other substances. In considering the 
 nature of the products which issue from volcanic fissures, 
 it must be remembered that many substances which 
 under ordinary circumstances do not exhibit marked 
 volatility are nevertheless easily carried away in fine 
 particles when a current of steam is passed over them. 
 As we shall have to point out in the sequel, different 
 volatile substances have a tendency to make their ap- 
 pearance at volcanic vents according as the intensity 
 of the action going on within it varies. 
 
 The volatile substances issuing from volcanic fis- 
 sures at high temperatures react upon one another, 
 and many new compounds are thus formed. We have 
 already seen how, by the action of sulphurous acid and 
 sulphuretted hydrogen on each other, the sulphur so 
 common in volcanic districts has been separated and 
 deposited. The hydrochloric acid acts very energetic- 
 ally on the rocks around the vents, uniting with, the 
 iron in them to form the yellow ferric-chloride. The 
 
GASES EMITTED FKOM VOLCANOES. 41 
 
 rocks all round a volcanic vent are not unfrequently 
 found coated with this yellow substance, which is almost 
 always mistaken by casual observers for sulphur. In 
 many volcanoes the constant passage through the rocks 
 of the various acid gases has caused nearly the whole of 
 the iron, lime, and alkaline materials of the rocks to be 
 converted into soluble compounds known as sulphates, 
 chlorides, carbonates, and borates ; and, on the removal 
 of these by the rain, there remains a white, powdery 
 substance, resembling chalk in outward appearance, but 
 composed of almost pure silica. There are certain 
 cases in which travellers have visited volcanic islands 
 where chemical action of this kind has gone on to such 
 an extent, that they have been led to describe the 
 islands as composed entirely of chalk. 
 
 Some of the substances issuing from volcanic vemX 
 such as hydrogen and sulphuretted hydrogen, are in- 
 flammable, and when they issue at a high temperature, 
 these gases burst into flame the moment that they 
 come into contact with the air. Hence, when volcanic 
 fissures are watched at night, faint lambent flames are 
 frequently seen playing over them, and sometimes these 
 flames are brilliantly coloured, through the presence of 
 small quantities of certain metallic oxides. Such vol- 
 canic flames, however, are scarcely ever strongly lumi- 
 nous and, as we have already seen, the red, glowing light 
 which is observed over volcanic mountains in eruption 
 is due to quite another cause. The study by the aid of 
 the spectroscope of the flames which issue from volcanic 
 
42 VOLCANOES. 
 
 vents promises to throw much new light on the rarer 
 materials ejected by volcanoes. Spectroscopic obser- 
 vations of this kind have already been commenced by 
 Janssen, at Stromboli and Santorin. 
 
 Some of the volatile substances issuing from volcanic 
 vents, are at once deposited when they come in contact 
 with the cool atmosphere, others form new compounds 
 with one another and the constituents of the atmo- 
 sphere, while others again attack the materials of the 
 surrounding rocks and form fresh chemical compounds 
 with some of their ingredients. Thus, there are con- 
 tinually accumulating on the sides and lips of volcanic 
 fissures deposits of sulphates, chlorides, and borates of 
 the alkalies and alkaline earths, with sal-ammoniac, sul- 
 phur, and the oxides and sulphides of certain metals. 
 The lips of the fissures from which steam and acid 
 gases issue in volcanoes are constantly seen to be coated 
 with yellow and reddish-brown incrustations, consisting 
 of mixtures, in varying proportions, of these different 
 materials, and these sometimes assume the form of 
 stalactites and pendent masses. 
 
 Some of these products of volcanic action are of 
 considerable commercial value. At Vulcano regular 
 chemical works have been established in the crater of 
 the volcano, by an enterprising Scotch firm, a great 
 number of workmen being engaged in collecting the 
 materials which are deposited around the fissures, and 
 are renewed by the volcanic action almost as soon as 
 they are removed. In fig. 6 I have given a sketch of 
 
DEPOSITS AEOUND VOLCANIC VENTS. 
 
 43 
 
 this singular spot, taken from the high ground of the 
 neighbouring Island of Lipari. From the village at 
 the foot of the volcano, where the workmen live, a zig- 
 zag road has been constructed leading up the side, and 
 down into the crater of the volcano. On this road, 
 workmen and mules, laden with the various volcanic 
 . materials, may be seen constantly passing up and down. 
 
 FIG. 6. VIEW OF VULCANO, WITH VULCANELLO IN THE FOREGROUND 
 TAKEN FROM THE SOUTH END OF THE ISLAND OF LlPARI. 
 
 Vulcano appears to have been frequently in a state of 
 violent eruption during the past 2,000 years the last 
 great outburst having taken place in 1786. In 1873 the 
 activity in the crater of Vulcano suddenly became more 
 pronounced in character, and the workmen hastened 
 to escape from the dangerous spot, but, before they 
 could do so, several of them were severely injured by 
 the explosions. After this outburst, which did not prove 
 
44 VOLCANOES. 
 
 to be of very violent character, the quantity of gases 
 issuing from the fissures in the crater was for a time 
 much greater than before, and the productiveness of 
 these great natural chemical works was proportionately 
 increased: but eventually the action died out almost 
 entirely. The chief products of Vulcano which are of 
 commercial value, are sal-ammoniac, sulphur, and boracic 
 acid. At one time it was even contemplated that great 
 leaden chambers should be erected over the principal 
 fissures at the bottom of the crater of Vulcano, in which 
 chambers the volatile materials might be condensed 
 and collected. The change in the condition of the 
 volcano has unfortunately prevented the carrying out 
 of this bold project. 
 
 Besides the volatile substances which issue from 
 volcanic vents, mingling with the atmosphere or con- 
 densing upon their sides, there are also many solid 
 materials ejected, and these may accumulate around 
 the orifices, till they build up mountains of vast dimen- 
 sions, like Etna, Teneriffe, and Chimborazo. Some of 
 these solid materials are evidently fragments of the 
 rock-masses, through which the volcanic fissure has 
 been rent ; these fragments have been carried upwards 
 by the force of the steam-blast and scattered over 
 the sides of the volcano. But the principal portion of 
 the solid materials ejected from volcanic orifices con- 
 sists of matter which has been extruded from sources 
 far beneath the surface, in a highly-heated and fluid or 
 semi-fluid condition. 
 
EJECTED ROCK-FKAG-MENTS. 46 
 
 The fragments torn from the sides of volcanic 
 fissures consist of the rocks through which the eruptive 
 forces may happen to have opened their way ; masses 
 of sandstone, limestone, slate, granite, &c., are thus 
 frequently found in considerable numbers among 
 materials which build up volcanic mountains. Thus, 
 some of the volcanic cones in the Eifel are very largely 
 made up of fragments of slate, which have been torn 
 from the sides of the vents by the uprushing masses of 
 steam. At Vesuvius fragments of limestone are fre- 
 quently ejected, and may be picked up all over the 
 slopes of the mountains. These limestone-fragments 
 frequently contain fossils, and Professor Guiscardi, of 
 Naples, has been able to collect several hundred species 
 of shells, transported thus by volcanic action from the 
 rock-masses which form the foundation of the volcano 
 of Vesuvius. The action of water at a high tempera- 
 ture, and under such enormous pressure as must 
 exist beneath volcanic mountains, has often produced 
 changes in the rocks of which fragments are ejected 
 from volcanic vents. The so-called 'lava' ornaments, 
 which are so extensively sold at Naples, are not made 
 from the materials to which geologists apply that 
 name, but from the fragments of altered limestone 
 that have been torn from the rocks beneath the 
 mountain, and scattered by the eruptive forces all over 
 its sides. The chemical action of the superheated 
 and highly-compressed steam on the rocks beneath 
 volcanoes frequently results in the formation of beauti- 
 
46 VOLCANOES. 
 
 fully crystallized minerals. Such crystallized minerals 
 abound in the rock-fragments scattered over the sides 
 of Vesuvius and other volcanoes, both active and ex- 
 tinct. They have been formed in the great chemical 
 laboratories which exist beneath the volcano, and have 
 been brought to the surface by the action of the steam- 
 jets issuing from its fissures. 
 
 Of still greater interest are those materials which 
 issue from volcanic orifices in an incandescent, and 
 often in a molten, condition, and which are evidently 
 derived from sources far below the earth's surface. It 
 is to these materials that the name of < lavas ' is 
 properly applied. 
 
 Lavas present a general resemblance to the slags 
 and clinkers which are formed in our furnaces and 
 brick-kilns, and consist, like them, of various stony 
 substances which have been more or less perfectly 
 fused. When we come to study the chemical com- 
 position and the microscopical structure of lavas, how- 
 ever, we shall find that there are many respects in which 
 they differ entirely from these artificial products. 
 
 Let us first consider the facts which are taught us 
 concerning the nature and origin of lavas, by a chemical 
 analysis of them. 
 
 Of the sixty-five or seventy chemical elements, only a 
 very small number occur at all commonly in lavas. Eight 
 elements, indeed, make up the great mass of all lavas 
 these are oxygen, silicon, aluminium, magnesium, 
 calcium, iron, sodium, and potassium. But even these 
 
CHEMICAL COMPOSITION OF LAVAS. 47 
 
 eight elements are present in very unequal propor- 
 tions. Oxygen makes up nearly one-half the weight of 
 all lavas. Almost all the other elements found in lavas 
 exist in combination with oxygen, so that lavas consist 
 entirely of what chemists call ' oxides.' This is a most 
 remarkable circumstance, which, as we shall presently 
 see, is of great significance. The metalloid silicon 
 makes up about one-fourth of the weight of most lavas, 
 and the metal aluminium about one-tenth. The other 
 five elements vary greatly in their relative proportions 
 in different lavas. 
 
 In all lavas the substance which forms the greatest 
 part of the mass is the compound of oxygen and silicon, 
 known as silica or silicic acid. In its pure form, this 
 substance- is familiar to us as quartz, or rock-crystal 
 and flint. Silica is present in all lavas in proportions 
 which vary from one-half to four-fifths of the whole 
 mass. Now, this substance, silica, has the property of 
 forming more complex compounds by uniting with the 
 other oxides present in lavas namely, the oxides of 
 aluminium, magnesium, calcium, iron, potassium, and 
 sodium. Silica is called by chemists an acid, the other 
 oxides in lavas are termed bases, and the compounds 
 of silica with the bases are known as silicates. Hence 
 we see that lavas are composed of a number of different 
 silicates the silicates of aluminium, magnesium, cal- 
 cium, iron, potassium, and sodium. 
 
 The above statements will perhaps be made clearer 
 by the accompanying table from which it will be seen 
 
48 
 
 VOLCANOES. 
 
 ELEMENTS BINARY COMPOUNDS 
 
 Acid Bases 
 
 
 'Silicon .... Silica- 
 
 
 1- 
 
 X 
 
 
 Aluminium 
 Magnesium 
 
 Alumina 
 Magnesia . 
 Lime . . . 
 Iron 
 
 Calcium 
 Iron 
 
 Potassium ...'.. 
 
 Potash . . 
 Soda . . . 
 
 ^Sodium 
 
 that lavas are compounds in varying proportions of six 
 kinds of salts namely, the silicates of alumina, mag- 
 nesia, lime, iron, potash, and soda. 
 
 COMPOSITION OF LAVAS. 
 
 SALTS 
 
 Silicate of Alumina 
 ,, Magnesia 
 Lime 
 ,, Iron 
 ; , Potash 
 Soda 
 
 Now, in some lavas the acid constituent, or silica, is 
 present in much larger proportions than in others. 
 Those lavas with a large proportion of silica are called 
 4 acid lavas,' those with a lower percentage of silica, 
 and therefore a higher proportion of the bases, are 
 known as the 'basic lavas.' It is convenient to employ 
 the term f intermediate lavas ' for those in which the 
 proportion of silica is lower than in the acid lavas, and 
 the proportion of the bases is lower than in the basic 
 lavas. 
 
 The acid lavas contain from 66 to 80 per cent, of 
 silica ; they are poor in lime, magnesia, and oxide of 
 iron, but rich in potash and soda. The basic lavas 
 contain from 45 to 55 per cent, of silica ; they are rich 
 in magnesia, lime, and oxide of iron, but poor in soda 
 
DIFFEKENT KINDS OF LAVA. 49 
 
 and potash. In the intermediate lavas the proportion 
 of silica varies from 55 to 66 per cent. 
 
 As the basic-lavas contain a larger proportion of 
 oxide of iron and other heavy oxides than the acid- 
 lavas, the former have usually a higher specific 
 gravity than the latter ; it is, indeed, possible in most 
 cases to distinguish between these different varieties 
 by simply weighing them in water and in air. 
 
 The basic lavas are usually of much darker colour 
 than the acid lavas the terms acid lavas, intermediate 
 lavas, and basic lavas correspond indeed pretty closely 
 with the names trachytes, greystones and basalt, which 
 were given to the varieties of lavas by the older writers 
 on volcanoes, at a time when their chemical constitution 
 had not been accurately studied. Fresh lavas of acid 
 composition are usually nearly white in colour, inter- 
 mediate lavas are of various tints of grey, and basic 
 lavas nearly black. It must be remembered, however, 
 that colour is one of the least persistent, and therefore 
 one of the least valuable, characters by means of which 
 rocks can be discriminated, and also that by exposure 
 to the influence of the atmospheric moisture the iron 
 present in all lavas is affected, and the lavas belonging 
 to all classes, when weathered, assume reddish and 
 reddish-brown tints. 
 
 Geologists have devised a great number of names 
 for the various kinds of lava which have been found 
 occurring round volcanic vents in different parts of the 
 world, and the study of these varieties is full of interest. 
 
50 VOLCANOES. 
 
 For our present purpose, however, it will be sufficient 
 to state that they nearly all fall into five great groups, 
 known as the Rhyolites, the Trachytes, the Andesites, 
 the Phonolites, and the Basalts. The Rhyolites are 
 acid lavas, the Basalts are basic lavas, and the Tra- 
 chytes, Andesites, and Phonolites, different kinds of 
 intermediate lavas, distinguished by the particular 
 minerals which they contain. 
 
 Before we part from this subject of the classification 
 of lavas according to their chemical composition, it will 
 be well to point out that there exists a small group of 
 lavas which stand quite by themselves, and cannot be 
 referred to either of the classes we have indicated. 
 They contain a smaller proportion of silica, and a much 
 larger proportion of magnesia and oxide of iron than 
 the other lavas, and may be made to constitute a small 
 sub-group, to which we may apply the term of ' ultra- 
 basic lavas.' Although much less widely distributed 
 than the other varieties, they are, in some respects, 
 as we shall presently have to point out, of far greater 
 interest to the geologist than all the other kinds of 
 lavas. 
 
 We will now proceed to consider the facts which 
 are brought to light concerning the nature of lavas, 
 when they are studied by the aid of the microscope. 
 Although most lavas appear at first sight to be opaque 
 substances, yet it is easy to prepare slices of them 
 which are sufficiently thin to transmit light. In such 
 thin transparent slices we are able to make out, by the 
 
MINUTE STRUCTURE OF LAVAS. 1 
 
 aid of the microscope, certain very interesting details 
 of structure, which afford new and important evidence 
 bearing on the mode of origin of these rocks. 
 
 Most lavas are capable of being melted by the heat 
 of our furnaces ; but the different kinds of lava vary 
 greatly in the degree of their fusibility. The basic 
 lavas, or those with the smallest proportion of silica, 
 are usually much more easily fusible than those which 
 contain a high percentage of silica, the acid lavas. 
 
 Now, it is a very noteworthy circumstance, that when 
 a lava is artificially fused it assumes on cooling very 
 different physical characters to those which were pre- 
 sented by the original rock. 
 
 If we examine the freshly-broken surface of a piece 
 of lava, we shall, in most cases, find that it contains a 
 great number of those regular-shaped bodies which we 
 call crystals ; in come cases these crystals are so small 
 as to be scarcely visible to the naked eye, in others 
 they may be an inch or more in length. Most lavas 
 are thus seen to be largely made up of crystals of 
 different minerals. The minerals which are usually 
 contained in lavas are quartz, the various kinds of 
 felspar, augite, hornblende, the different kinds of mica, 
 olivine, and magnetite. 
 
 But when a piece of lava is melted in a furnace, all 
 these crystalline minerals disappear, and the resulting 
 product is the homogeneous substance which we call 
 glass. If, as many suppose, lavas acquire the fluidity 
 which they possess when issuing from volcanic vents 
 
52 VOLCANOES. 
 
 as the result of simple fusion, it is strange that arti- 
 ficially fused lavas do not agree more closely in cha- 
 racter with the natural products. 
 
 A careful examination of different kinds of lavas, 
 however, will show that they vary very greatly in 
 character among themselves. Some lavas are as per- 
 fectly glassy in structure as those which have been 
 artificially fused, while others contain great numbers 
 of crystals, which may sometimes be of very large size. 
 
 If we prepare thin transparent slices of these dif- 
 ferent kinds of lavas, and examine them by the aid 
 of the microscope, we shall find that lavas are made 
 up of two kinds of materials, a base or groundmass ol 
 a glassy character, and distinct crystals of different 
 minerals, which are irregularly distributed through this 
 glassy base, like the raisins, currants, and pieces of 
 candied peel in a cake. In some cases the glassy base 
 makes up the whole mass of the rock; in others, 
 smaller or larger numbers of crystals are seen to be 
 scattered through a glassy base ; while in others again 
 the crystals are so numerous that the presence of an 
 intervening glassy base or groundmass can only be 
 detected by the aid of the microscope. 
 
 If thin slices of the glassy materials of lavas be 
 examined with high magnifying powers, new and in- 
 teresting facts are revealed. Through the midst of the 
 clear glassy substance cloudy patches are seen to be 
 diffused; and, if we examine them with a still higher 
 power, these cloudy patches resolve themselves into 
 
STUDY OF LAVAS WITH THE MICROSCOPE. 53 
 
 innumerable particles, some transparent and others 
 opaque, having very definite outlines. At the same 
 time fresh cloudy patches are brought into view, which 
 can only be resolved by yet higher powers of the 
 microscope. In examining these natural glasses by 
 the aid of the microscope, we are forcibly reminded of 
 what occurs when the 6 Milky Way ' and some other 
 parts of the heavens are studied with a telescope. As 
 the power of the instrument is increased the nebulous 
 patches are resolved into distinct stars, but fresh nebu- 
 lous masses come into view, which are in turn resolved 
 into stars, when higher powers of the instrument are 
 employed. 
 
 In the Frontispiece, No. 1 illustrates the appearance 
 presented by these volcanic glasses when examined 
 with a high power of a microscope. Through a glassy 
 base is seen a number of diffused nebulous patches, 
 which are in places resolved into definite particles. 
 
 These minute particles of definite form, which the 
 microscope has revealed in the midst of the glassy 
 portions of lava, have received the name of microliths, 
 or crystallites. The study of the characters and mode 
 of arrangement of these microliths or crystallites has 
 in recent years thrown much new light on the interest- 
 ing problems presented by lavas. 
 
 In some glassy lavas the microliths or crystallites, 
 instead of being indiscriminately diffused through the 
 mass of the base or groundmass, are found to be col- 
 lected together into groups of very definite form. In 
 
54 VOLCANOES. 
 
 No. 2 of the Frontispiece we have a section of a glassy 
 rock in which the crystallites have united together, so 
 as to build up groups presenting the most striking 
 resemblance to fronds of ferns. Around these groups 
 spaces of clear glass have been left by the gathering 
 up of the crystallites, which in other parts of the mass 
 are seen to be equally diffused through it. In this 
 formation of groups of microliths we cannot but re- 
 cognise the action of those ciystalline forces, which on 
 frosty mornings cover our windows with a mimic vege- 
 tation composed of icy particles. 
 
 In other cases, again, the crystallites scattered 
 through the glassy portions of lavas unite in radial 
 groups about certain centres, and thus build up globu- 
 lar masses to which the name of ' sphserulites ' has been 
 given. No. 3 in the Frontispiece illustrates the forma- 
 tion of these sphserulites. 
 
 Now, a careful study of the microliths or crystal- 
 lites has proved that they are the minute elements of 
 which those wonderfully beautiful objects which we 
 call crystals are built up. In some cases we can see 
 that the crystallites are becoming united together in 
 positions determined by mathematical laws, and the 
 group is gradually assuming the outward form and in- 
 ternal structure of a crystal. In other cases crystals 
 may be found which are undergoing a disintegrating 
 action, and are then seen to be made up of minute 
 elements similar to the crystallites or microliths of 
 glassy rocks. 
 
CKYSTALLITES AND CRYSTALS. 55 
 
 This conclusion is confirmed by the fact that if we 
 take an artificially fused lava and allow it to cool slowly, 
 it will be found that the glassy mass into which it has 
 resolved itself contains numerous crystallites. If the 
 cooling process be still further prolonged, these crystal- 
 lites will be found to have united themselves into 
 definite groups, and sometimes distinct crystals are 
 formed in the mass; under these circumstances the 
 rock frequently loses its glassy appearance and assumes 
 a stony character. 
 
 In connection with this. subject, it may be men- 
 tioned that some years ago a very ingenious invention 
 was submitted to trial in the works of the Messrs. 
 Chance, of Birmingham. It had been suggested that 
 if certain lav r as of easy fusibility were melted and 
 poured into moulds, we might thus obtain elaborately 
 ornamented stone-work, composed of the hardest 
 material, without the labour of the mason. The molten 
 rock when quickly cooled was found to assume the 
 form of a black glass, but when very slowly cooled 
 passed into a stony material. Unfortunately, it was 
 found that this material did not withstand the weather 
 like ordinary building stones, and, in consequence, the 
 manufacture had to be abandoned. 
 
 Now, the study of the products of volcanoes has led 
 geologists to recognise the true relations between 
 glassy and crystalline rocks. 
 
 In the amorphous mixture of various silicates which 
 compose a glass, chemical affinity causes the separation 
 
56 VOLCANOES. 
 
 of certain portions of definite composition, and these 
 form the microliths or elements of which different 
 crystalline minerals are built up. Under the influence 
 of the crystalline forces, there is a great shaking or 
 agitation in the mass, and the microliths of similar kind 
 come together and become united, like the fragments 
 in Ezekiel's valley of dry bones. 
 
 Although we cannot see this process taking place 
 under our eyes, in a mass of lava, yet we may study 
 specimen? in which the action has been arrested in 
 its different stages. In order to understand the 
 development of an acorn into an oak-tree, it is not 
 necessary to watch the whole series of changes in a 
 particular case. A visit to an oak-thicket, in which 
 illustrations of every stage of the transformation may 
 be found, will afford us equally certain information on 
 the subject. 
 
 In the same way by the examination of such a 
 series of rock-sections as that represented in the Fron- 
 tispiece, we may understand how, in the midst of a 
 mass of mixed silicates constituting a natural glass, 
 the separation of microliths takes place ; these unite 
 into groups which are the skeletons of crystals, and 
 finally, by the filling up of the empty spaces in these 
 skeletons, complete crystals are built up. The series 
 of operations may, however, be interrupted at any 
 stage, and this stage we may have the chance of 
 studying. 
 
 We are able, as we shall show in a future chapter, 
 
GLASSY AND CRYSTALLINE LAVAS. 57 
 
 to examine many rock-masses that have evidently 
 formed the reservoirs from which volcanoes have been 
 supplied, and others that fill up the ducts which con- 
 stituted the means of communication between these 
 subterranean reservoirs, and the surface of the earth. 
 Now in these subterranean regions the lavas have been 
 placed under conditions especially favourable for the 
 action of the crystalline forces they must have cooled 
 with extreme slowness, and they must have been under 
 an enormous pressure, produced in part by the weight 
 of the superincumbent rocks, and in part by the ex- 
 pansive force of the imprisoned steam. We are not, 
 therefore, surprised to find that in these subterranean 
 regions, the lavas, while retaining the same chemical 
 composition, have assumed a much more perfectly 
 crystalline condition. In some cases, indeed, the whole 
 rock has become a mass of crystals without any base 
 or groundmass at all. 
 
 An examination of the Frontispiece will illustrate 
 this perfect gradation from the glassy to the crystalline 
 condition of lavas. No. 1 represents a glass through 
 which microliths or crystallites of different dimensions 
 and character are diffused. In Nos. 2 and 3, these crys- 
 tallites have united to form regular groups. In No. 4, 
 which may be taken as typical of the features presented 
 by most lavas, we have a glassy groundmass containing 
 microliths (a ' crypto-crystalline base'), through which 
 distinct crystals are distributed, Nos. 5 and 6 illustrate 
 the characters presented by lavas which have consoli- 
 4 
 
58 VOLCANOES. 
 
 dated at considerable depths beneath the surface ; in 
 the former we have a mass of small crystals (a * micro- 
 crystalline base ') with larger crystals scattered through 
 it ; while the latter is entirely made up of large 
 crystals without any trace of a base or groundmass. 
 
 Now, as all lavas are found sometimes assuming the 
 glassy condition at the surface, so when seen in the 
 masses which have consolidated with extreme slowness, 
 and under great pressure, in subterranean regions, the 
 same materials are found in the condition of a rock 
 which is built up entirely of crystals. Chemists have 
 found that artificial mixtures of silicates in which soda 
 and potash are present in considerable quantities, have 
 a great tendency to assume the glassy condition on cool- 
 ing from a state of fusion, and glass manufacturers are 
 always careful to use considerable proportions of the 
 alkalis as ingredients, in making glass. It is found, in 
 like manner, that those lavas which contain the largest 
 portion of the silicates of soda and potash (the ' acid 
 lavas ') most frequently assume the condition of a 
 natural glass. 
 
 Geologists have given distinct names to the glassy 
 and the perfectly crystalline conditions of the different 
 kinds of lavas, the glassy varieties being found in 
 masses which have cooled rapidly near the surface, and 
 the crystalline varieties in masses which have cooled 
 slowly at great depths. The names of these two con- 
 ditions of the five great classes into which we have 
 divided lavas are as follows : 
 
HIGHLY CRYSTALLINE IGNEOUS KOCKS. 59 
 
 Crystalline Forms. Lavas. Glassy Forms. 
 
 Granite .... Ehyolite \ 
 
 Syenite .... Trachyte / Obsidian. 
 
 Diorite .... Andesite f 
 
 Miascite .... PhonoliteJ 
 
 Gabbro .... Basalt Tachylyte. 
 
 As vitreous rocks have little in their general appear- 
 ance to distinguish them from one another, the glassy 
 forms of the first four classes of lava have not hitherto 
 received distinct names, but have been confounded 
 together under the name of obsidian. If we deter- 
 mine the specific gravities of rocks having the same 
 composition but different structures, we shall find that 
 they become heavier in proportion as the crystalline 
 structure is developed in them. Thus gabbro is 
 heavier, but tachylyte is lighter than basalt, bulk for 
 bulk, though all have the same chemical composition. 
 
 Nor are the crystals contained in lavas less worthy 
 of careful study, by the aid of the miscroscope, than 
 the more or Jess glassy groundmass in which they are 
 embedded. Mr. Sorby has shown that the crystals 
 found in lavas, exhibit many interesting points of 
 difference from those which separate out in the midst 
 of a mass of the same rock, when it has been artificially 
 melted and slowly cooled. There are other facts which 
 also point to the conclusion that, while the glassy 
 groundmass of lavas may have been formed by cooling 
 from a state of fusion, the larger and well-formed 
 crystals in these lavas must have been formed under 
 other and very different conditions. 
 
60 VOLCANOES. 
 
 The larger crystals in lavas exhibit evidence of 
 having been slowly built up in the midst of a glassy 
 mass, containing crystallites and small crystals. We 
 can frequently detect evidence of the interruptions 
 which have occurred in the growth of these crystals in 
 the concentric zones of different colour or texture 
 which they exhibit ; and portions of the glassy base or 
 groundmass are often found to have been caught up 
 and enclosed in these crystals during their growth. 
 
 But when we find, as in the porphyritic pitchstones, 
 a glassy base containing only minute crystallites, 
 through which large and perfectly formed crystals are 
 distributed, we can scarcely doubt that the minute 
 crystallites and the larger crystals have separated from 
 the base under very different conditions. This is indi- 
 cated by the fact that we detect in these cases no con- 
 necting links between the embryo microliths and the 
 perfect crystals ; and a confirmation of the conclusion 
 is seen in the circumstance that many of the crystals 
 are found to have suffered injury as if from transport, 
 their edges and angles being rounded and abraded, and 
 portions being occasionally broken off from them. 
 
 Hence we are led to conclude that the larger crystals 
 in lavas were probably separated from the amorphous 
 mass in the subterranean reservoirs beneath the volcano, 
 and were floated up to the surface in the midst of the 
 liquefied glassy material which forms the groundmass 
 of lavas. When we come to examine these crystals 
 more closely, we find that certain very curious phe- 
 
FIG. 7. MINUTE CAVITIES, CONTAINING LIQUIDS, IN THE 
 CRYSTALS OP KOCKS. 
 
LIQUID CAVITIES IN CEYSTALS. 61 
 
 nomena are exhibited by them which lend powerful 
 support to this conclusion. 
 
 It is found convenient by geologists to designate 
 those rocks which have consolidated in deep-seated 
 portions of the earth's crust as Plutonic Eocks, con- 
 fining the name of Volcanic rocks to those consolidating 
 at the surface ; but Plutonic and Volcanic Rocks shade 
 into one another by the most insensible gradations. 
 
 When the crystals embedded in granitic rocks, and 
 in some lavas, are examined with the higher powers of 
 the microscope, they are frequentl} seen to contain great 
 numbers of excessively minute cavities. Each of these 
 cavities resembles a small spirit-level, having a quantity ' 
 of liquid and a bubble of gas within it. In fig. 7 we 
 have given a series of drawings of these cavities in 
 crystals as seen under a high power of the microscope. 
 In No. 1 a group of such cavities is represented, one of 
 which is full of liquid, while two others are quite empty; 
 the remaining cavities all contain a liquid with a 
 moving bubble of gas. In No. 2 two larger cavities are 
 shown, containing a liquid and a bubble of gas ; and it 
 will be seen from these how varied in form these 
 cavities sometimes are. In Nos. 3, 4 and 5 the liquid 
 in the cavities contains, besides the bubbles, several 
 minute crystals ; and in No. 6 we have a cavity con- 
 taining two liquids and a bubble. 
 
 In the largest of such cavities the bubble is seen to 
 change its place so as always to lie at the upper side of 
 the cavity, when the position of the latter is altered, just 
 
62 
 
 VOLCANOES. 
 
 as in a spirit-level. But in the smallest cavities the 
 bubbles appear to be endowed with a power of sponta- 
 neous movement ; like imprisoned creatures trying to 
 escape, these bubbles are seen continually oscillating 
 from side to side and from end to end of the cavities 
 which enclose them. In fig. 8 a minute cavity con- 
 taining a liquid and bubble is shown, the path pursued 
 by the latter in its wonderful gyrations being indicated 
 by the dark line. These cavities are exceedingly mi- 
 
 FIG. 8. MINUTE LIQUID-CAVITY IN A CRYSTAL, WITH A MOVING 
 BUBBLE. (The path of the bubble is indicated by the dark line.) 
 
 nute, and so numerous that in some crystals there 
 must be millions of them present ; indeed, in certain 
 cases, as we increase the magnifying power of our micro- 
 scopes, new and smaller cavities continually become 
 visible. It has been estimated that in some instances 
 the number of these minute liquid-cavities in the crys- 
 tals of rocks amounts to from one thousand millions 
 to ten thousand millions in a cubic inch of space. 
 
 What is the nature "of the liquids which are thus 
 imprisoned in these cavities contained in the crystals of 
 
NATUKE OF LIQUIDS IN CAVITIES. 63 
 
 lavas and granites ? Careful experiments have given 
 a conclusive answer to this question. In many cases 
 the liquid is water, usually containing considerable 
 quantities of saline matter dissolved in it. Some- 
 times the saline matters are present in such abund- 
 ance that they cannot all pass into solution, but 
 crystallize out, as in fig. 7 Nos. 3, 4, 5 where cubic 
 crystals of the chlorides of sodium and potassium are 
 seen floating in the liquid ; in other cases the liquid is 
 a hydrocarbon like the mineral oil which is present in 
 great abundance in deep-seated rocks in many parts of 
 the globe. But in some other cases the liquid con- 
 tained in the cavities of crystals is found to be one 
 which could scarcely be anticipated to occur under 
 such circumstances the gas known as carbonic acid, 
 which under extreme pressure can be reduced to a 
 liquid condition. In cavities containing liquefied car- 
 bonic acid, if the rock be warmed up to 86 or 90 Fahr- 
 enheit the bubble suddenly vanishes, sometimes with 
 an appearance like ebullition or boiling, as represented 
 in fig. 9. Now the temperature which we have in- 
 dicated is the 'critical point' of carbonic acid, and 
 above that temperature it cannot exist in a liquid con- 
 dition, however great may be the pressure to which it 
 is subjected. The liquid has been converted into a 
 gas which completely fills the cavity. The carbonic 
 acid in the cavities of crystals has frequently been 
 isolated and its nature placed beyond doubt by spec- 
 troscopic and ordinary chemical tests. 
 
64 VOLCANOES. 
 
 The presence of these liquids in the cavities of 
 crystals clearly proves that the latter must have been 
 formed under enormous pressure a pressure sufficiently 
 great to reduce, not only steam, but also volatile hydro- 
 carbons and even gaseous carbonic acid, to the bulk of 
 a liquid. 
 
 Such conditions of enormous pressure we may infer 
 
 FIG. 9. CAVITY IN CRYSTAL CONTAINING CARBONIC-ACID GAS AT A 
 TEMPERATURE OK 86 F., AND PASSING FROM THE LIQUID TO THE 
 GASEOUS CONDITION. 
 
 to exist in the deep-seated reservoirs beneath volcanoes, 
 where, besides the weight of the superincumbent rock- 
 masses, we have the compressing force of great quantities 
 of elastic vapour held in confinement. The crystals of 
 which granitic rocks are entirely built up exhibit clear 
 
CAUSE OF MOVEMENT OF BUBBLES. 65 
 
 evidence of having been all formed under these con- 
 ditions of enormous pressure. The glassy "base or 
 groundmass of lavas, on the other hand, presents all 
 the characters of materials that have cooled from a 
 state of fusion. Most lavas consist in part of crystals, 
 exhibiting fluid-cavities like those present in granite, 
 and in part of a base, which has evidently been formed 
 by the cooling of a fused mass. We are therefore 
 justified in concluding that the crystals have been 
 formed in subterranean recesses, and that the ground- 
 mass or base has consolidated at the surface. The 
 bearing of these conclusions upon some of the great 
 problems presented by volcanoes we shall have occasion 
 to point out in the sequel. 
 
 One of the most interesting inquiries suggested by 
 the study of the liquid-cavities in volcanic rocks is 
 that of the cause of the apparently spontaneous move- 
 ment of the bubbles which we have described as taking 
 place in some of the smaller of them. The ingenious 
 experiments of Mr. Noel Hartley have suggested to 
 Professor Stokes an explanation which is probably the 
 true one. It appears that these minute globes of 
 vapour are in such a state of unstable equilibrium as 
 to be affected by the smallest changes of temperature, 
 and that the variations in the heat of the atmosphere, 
 due to currents of air and the movement of warm 
 or cold bodies through it, are sufficient to cause the 
 oscillation of these sensitively poised bubbles. 
 
 The short account which we have been able to give 
 
66 VOLCANOES. 
 
 in the foregoing pages of the researches that have been 
 carried on concerning the nature of the materials ejected 
 from volcanoes will serve to show that these investiga- 
 tions have already made known .many facts of great 
 interest, and that the further pursuit of them is full of 
 the highest promise. To the scientific worker no sub- 
 ject is too vast for his research, no object so minute as 
 to be unworthy of his most patient study. In some of 
 our future inquiries concerning the nature of volcanic 
 action, we shall be led to an investigation of the phe- 
 nomena displayed in the sun, moon, comets and other 
 great bodies of the universe ; but another road to truths 
 of the same grandeur and importance is found, as we 
 have seen, in an examination of the mode of develop- 
 ment of crystallites, and a study of the materials 
 contained in the microscopic cavities of the minutest 
 crystals. 
 
CHAPTEK IV. 
 
 THE DISTRIBUTION OF THE MATERIALS EJECTED FROM 
 VOLCANIC VENTS. 
 
 THE escape of great quantities of steam and other gases 
 from the midst of a mass of fluid or semi-fluid lava 
 gives rise to the formation of vast quantities of froth 
 or foam upon its surface. This froth or foam, which 
 is formed upon the surface of lava by the escape of 
 gaseous matters from within it, is made up of portions 
 of the lava distended into vesicles, in the same way 
 that bubbles are formed on the surface of water. It 
 bears precisely the same relation to the liquid mass of 
 lava that the white crest of foam upon an advancing 
 wave does to the sea-water, from the bubbles of which 
 it is formed. 
 
 This froth upon the surface of lavas varies greatly 
 in character according to the nature of the material 
 from which it is formed. In the majority of cases the , 
 lavas consist, as we have seen, of a mass of crystals 
 floating in a liquid magma, and the distension of such 
 a mass by the escape of steam from its midst gives rise 
 to the formation of the rough cindery-looking material 
 
68 VOLCANOES. 
 
 to which the name of ' scoria ' is applied. But when 
 the lava contains no ready-formed crystals, but consists 
 entirely of a glassy substance in a more or less perfect 
 state of fusion, the liberation of steam gives rise to the 
 formation of the beautiful material known as 4 pumice.' 
 Pumice consists of a mass of minute glass bubbles ; 
 these bubbles have not usually, however, retained their 
 globular form, but have been elongated in one direction 
 through the movement of the mass while it was still in 
 a plastic state. 
 
 The steam frequently escapes from lava with such 
 violence that the froth or scum on its surface is broken 
 up and scattered in all directions, as the foam crests of 
 waves are dispersed by the wind during a storm. In 
 this way fragments of scoria or pumice are often thrown 
 to the height of many hundreds or thousands of feet 
 into the atmosphere, as we have seen is the case at 
 Stromboli and Vesuvius. Indeed, during violent erup- 
 tions, a continuous upward discharge of these fragments 
 is maintained, the ragged cindery masses hurtling one 
 another in the atmosphere, as they are shot perpen- 
 dicularly upwards to an enormous height and fall back 
 into the vent ; or they may rise obliquely and describe 
 curves so as to descend outside the orifice from which 
 they were ejected. 
 
 During their upward discharge and downward fall, 
 the cindery fragments are by attrition continually 
 reduced to smaller dimensions. The noise made by 
 these fragments, as they strike against one another in 
 
FINENESS OF VOLCANIC DUST. 69 
 
 the air during their rise and fall, is one of the most 
 noteworthy accompaniments of volcanic eruptions. It 
 has been noticed that in many cases there is a constant 
 diminution in the size of the fragments ejected during 
 a volcanic outburst, this being doubtless due to the 
 friction of the masses as they are ejected and re-ejected 
 from the vent. Thus it is related by Mr. Poulett 
 Scrope, who watched the Vesuvian eruption of 1822, 
 which lasted for nearly a month, that during the earlier 
 stages of the outburst fragments of enormous size were 
 thrown out of the crater, but by constant re-ejection 
 these were gradually reduced in size, till at last only 
 the most impalpable dust issued from the vent. This 
 dust filled the atmosphere, producing in the city of 
 Naples ' a darkness that might be felt,' and so exces- 
 sively finely divided was it, that it penetrated into all 
 drawers, boxes, and the most closely fastened receptacles, 
 filling them completely. Mr. Whymper relates that, 
 while standing on the summit of Chimborazo, he wit- 
 nessed an eruption of Cotopaxi, which is distant more 
 than fifty miles from the former mountain. The fine 
 volcanic dust fell in great quantities around him, and 
 he estimated that no less than two millions of tons 
 must have been ejected during this slight outburst. 
 Professor Bonney has examined this volcanic dust from 
 Cotopaxi, and calculates that it would take from 4,000 
 to 25,000 particles to make up a grain in weight. 
 
 Various names have been given by geologists to 
 the fragments ejected from volcanic vents, which, as we 
 
70 VOLCANOES. 
 
 have seen, differ greatly in their dimensions and other 
 characters. Sometimes masses of more or less fluid 
 lava are flung bodily to a great height in the atmo- 
 sphere. During their rise and fall these masses are 
 caused to rotate, and in consequence assume a globular 
 or spheroidal form. The water imprisoned in these 
 masses, during their passage through the atmosphere, 
 tends to expand into steam, and they become more or 
 less completely distended with bubbles. Such masses, 
 which sometimes assume very regular and striking 
 forms, are known as ' volcanic bombs.' Many volcanic 
 bombs have a solid nucleus of refractory materials. The 
 large, rough, angular, cindery-looking fragments "are 
 termed ' scoriae.' When reduced to the dimensions of 
 a marble or pea they are usually called by the, Italian 
 name of 'lapilli.' The still finer materials are known 
 as volcanic sand and dust. 
 
 There are, however, two names which are frequently 
 applied to these fragmentary materials ejected from 
 volcanoes, which are perhaps liable to give rise to mis- 
 conception. These are the terms 6 cinders ' and 6 ashes.' 
 It must be remembered that the scoriae or cindery- 
 looking masses are not, like the cinders of our fires, the 
 product of the partial combustion of a material contain- 
 ing inflammable gases, but are, like the clinkers of 
 furnaces and brick-kilns, portions of partially vitrified 
 and fused rock distended by gases. So, too, volcanic 
 ashes only resemble the ashes of our grates in being 
 very finely divided; they are not, like the latter, 
 
VOLCANIC BOMBS AND PELE'S HAIR. 71 
 
 the incombustible residue of a mass which has been 
 burnt. 
 
 The glassy lavas, when distended by escaping gases, 
 give rise to the formation of pumice, the white colour 
 of which, as in the case of the foam of a wave, is due 
 to the reflection of a portion of the light in its frequent 
 passage from one medium to another in this case from 
 air to glass, and from glass to air. The volcanic bombs 
 formed from glassy lavas are often of especially beauti- 
 ful and regular forms. Sometimes the passage of steam 
 through a mass of molten glass produces large quantities 
 of a material resembling spun glass. Small particles or 
 shots of the glass are carried into the air and leave 
 behind them thin, glassy filaments like a tail. At the 
 volcano of Kilauea in Hawaii this filamentous vol- 
 canic glass is abundantly produced, and is known as 
 1 Pele's Hair ' - Pele being the name of the goddess of 
 the mountain. Birds' nests are sometimes found com- 
 posed of this beautiful material. In recent years an 
 artificial substance similar to this Pele's hair has been 
 extensively manufactured by passing jets of steam 
 through the molten slag of iron-furnaces ; it resembles 
 cotton-wool, but is made up of fine threads of glass, 
 and is employed for the packing of boilers and other 
 purposes. 
 
 The very finely-divided volcanic dust is often borne 
 to enormous distances from the volcano out of which 
 it has been ejected. The force of the steam-current 
 carrying the fragments into the atmosphere is often so 
 
72 VOLCANOES. 
 
 great that they rise to the height of several miles above 
 the mountain. Here they may actually pass into the 
 upper currents of the atmosphere and be borne away to 
 the distance of many hundreds or thousands of miles. 
 Hence it is not an unusual circumstance for vessels at 
 sea to encounter at great distances from land falling 
 showers of this finely divided, volcanic dust. We some- 
 times meet with this far-travelled, volcanic dust under 
 very unexpected circumstances. Thus, in the spring 
 of 1875 I had occasion to visit Prof. Vom Eath of Bonn, 
 who showed me a quantity of fine volcanic dust which 
 had during the past winter fallen in considerable quan- 
 tities in certain parts of Norway. This dust, upon 
 microscopic examination, proved to be so similar to what 
 was known to be frequently ejected from the Icelandic 
 volcanoes that a strong presumption was raised that 
 volcanic outbursts had been going on in that island. 
 On returning to England I found that the first steamer 
 of the season had just reached Leith from Iceland, 
 bringing the intelligence that very violent eruptions 
 had taken place during the preceding months. 
 
 This finely-divided volcanic dust is thus carried by 
 the winds and spread over every part of the ocean. 
 Everyone is familiar with the fact that pumice floats 
 upon water ; this it does, not because it is a material 
 specifically lighter than water, but because cavities 
 filled with air make up a great part of its bulk. If 
 we pulverise pumice, we find the powder sinks readily 
 in water, but the rock in its natural condition floats 
 
DISPEKSION OF PUMICE AND VOLCANIC DUST. 73 
 
 for the same reason that an iron ship does because of 
 the air-chambers which it encloses. When this pumice 
 is ejected from a volcano and falls into a river or the 
 ocean, it floats for a long time, till decomposition 
 causes the breaking down of the thin glassy partitions 
 between the air chambers, and causes the admission of 
 water into the latter, by which means the whole mass 
 gets water-logged. Near the Liparis and other vol- 
 canic islands the sea is sometimes covered with frag- 
 ments of pumice to such an extent that it is difficult 
 for a boat to make progress through it, and the same 
 substance is frequently found floating in the open ocean 
 and is cast up on every shore. 
 
 During the year 1878 masses of floating pumice 
 were reported as existing in the vicinity of the Solomon 
 Isles, and covering the surface of the sea to such extent 
 that it took ships three days to force their way through 
 them. Sometimes these masses of pumice accumu- 
 late in such quantities along coasts that it is difficult 
 to determine the position of the shore within a mile or 
 two, as we may land and walk about on the great float- 
 ing raft of pumice. Now, recent deep-sea soundings, 
 carried on in the ' Challenger' and other vessels, have 
 shown that the bottom of the deepest portion of the 
 ocean, far away from the land, is covered with these 
 volcanic materials which have been carried through the 
 air or floated on the surface of the ocean. To these 
 deeper parts of the ocean no sediments carried down by 
 the rivers are borne, and the remains of calcareous 
 
74 VOLCANOES. 
 
 organisms are, in these abysses, soon dissolved ; under 
 such conditions, therefore, almost the only material 
 accumulating on the sea bottom is the ubiquitous wind- 
 ar?.d wave-borne volcanic products. These particles of 
 volcanic dust and fragments of pumice by their dis- 
 integration give rise to a clayey material, and the 
 oxidation of the magnetite, which all lavas contain, 
 communicates to the mass a reddish tint. This ap- 
 pears to be the true origin of those masses of 'red-clay' 
 which, according to recent researches, are found to 
 cover all the deeper parts of the ocean, but which 
 probably attain to no great thickness. 
 
 But while some portion of the materials ejected 
 from volcanoes may thus be carried by winds and waves, 
 so as to be dispersed over every part of the land and 
 the ocean-bed, another, and in most cases by far the 
 largest, portion of these ejections falls around the vol- 
 canic vent itself. It is by the constant accumulation 
 of these ejected materials that such great mountain 
 masses as Etna, Teneriffe, Fusiyama, and Chimborazo 
 have been gradually built up around centres of volcanic 
 action. 
 
 There are cases in which the formation of volcanic 
 mountains on a small scale has actually been observed 
 by trustworthy witnesses. There are other cases in 
 which volcanic mountains of larger size can be shown 
 to have increased in height and bulk by the fall upon 
 their sides and summits of fragmentary materials ejected 
 from the volcanic vent. In all cases the examination 
 
FOKMATION OF VOLCANIC MOUNTAINS. 75 
 
 of these mountain-masses leads to the conclusion that 
 they are entirely built up of just such materials as we 
 constantly see thrown out of volcanoes during eruption. 
 
 Thus we are led to the conclusion that all volcanic 
 mountains are nothing but heaps of materials ejected 
 from fissures in the earth's crust, the smaller ones 
 having been formed during a single volcanic outburst, 
 the larger ones being the result of repeated eruptions 
 from the same orifice which may, in some cases, have 
 continued in action for tens or hundreds of thousands 
 of years. 
 
 No observer has done such useful work in connection 
 with the study of the mode of formation of volcanic 
 mountains as our countryman, Sir William Hamilton, 
 who was ambassador at Naples from 1764 to 1800, and 
 made the best possible use of his opportunities for ex- 
 amining the numerous volcanoes in Southern Italy. 
 
 A little to the west of the town of Puzzuoli on the 
 Bay of Naples there stands a conical hill rising to the 
 height of 440 feet above the level of the Mediterranean, 
 and covering an area more than half a mile in diameter. 
 Now we have the most conclusive evidence that in 
 ancient times no such hill existed on this site, which 
 was partly occupied by the Lucrine Lake, and the fact 
 is recognised 'in the name which the hill bears, that of 
 MonteJNuovo, or the 4 New Mountain.' See fig. 10. 
 
 Sir William Hamilton rendered admirable- service 
 to science by collecting all the contemporary records 
 relating to this interesting case, and he was able to 
 
76 VOLCANOES. 
 
 prove, by the testimony of several intelligent and trust- 
 worthy witnesses, that during the week following the 
 29th of September, 1538, this hill had gradually been 
 formed of materials ejected from a volcanic vent which 
 had opened upon this site. 
 
 The records collected by Hamilton with others which 
 have been discovered since his death prove most conclu- 
 sively the following facts. During more than two years, 
 the country round was affected by earthquakes, which 
 
 FIG. 10. MONTE Nuovo (440 FT. HIGH) ox THE SHORES OF THE BAY OF 
 NAPLES. 
 
 gradually increased in intensity and attained their 
 climax in the month of September 1538 ; on the 27th 
 and 28th of that month these earthquake shocks are said 
 to have been felt almost continuously day and night. 
 About 8 o'clock on the morning of the 29th, a depres- 
 sion of the ground was noticed on the site of the future 
 hill, and from this depression, water, which was at first 
 cold and afterwards tepid, began to issue. Four hours 
 afterwards the ground was seen to swell up and open, 
 forming a gaping fissure, within which incandescent 
 
HISTORY OF THE FORMATION OF MONTE NUOVO. 77 
 
 matter was visible. From this fissure numerous masses 
 of stone, some of them < as large as an ox,' with vast 
 quantities of pumice and mud, were thrown up to a 
 great height, and these falling upon the sides of the 
 vent formed a great mound. This violent ejection of 
 materials continued for two days and nights, and on 
 the third day a very considerable hill was seen to have 
 been built up by the falling fragments, and this hill 
 was climbed by some of the eye-witnesses of the erup- 
 tion. The next day the ejections were resumed, and 
 many persons who had ventured on the hill were in- 
 jured, and several killed by the falling stones. The 
 later ejections were however of less violence than the 
 earlier ones, and seem to have died out on the seventh 
 or eighth day after the beginning of the outburst. 
 The great mass of this considerable hill would appear, 
 according to the accounts which have been preserved, 
 to have been built up by the materials which were 
 ejected during two days and nights. 
 
 Monte Nuovo is a hill of truncated conical form, 
 which rises to the height of 440 feet above the waters 
 of the Mediterranean, and is now covered with thickets 
 
 of stone-pine.. The hill is entirely made up of volcanic 
 
 scoriae, lapilli, and dust, and the sloping sides have 
 evidently been produced_by these fragmentary materials 
 sliding over one another till they attained the angle of 
 rest ; just as happens with the earth and stone_s tipped 
 from railway-waggons during the construction of an 
 embankment. In the centre of this conical hill is a 
 
78 
 
 VOLCANOES. 
 
 vast circular depression, with steeply sloping sides, 
 which is of such depth that its bottom is but little 
 above the sea-level. This cup-shaped depression is 
 the ' crater ' of the volcano, and it has evidently been 
 formed by the explosive action which has thrown out 
 the materials immediately above the vent, and caused 
 them to be accumulated around it. 
 
 FlG. 11. MAP OF THE DISTRICT AROUND NAPLES, SHOWING MOXTE 
 NUOVO AND THE SURROUNDING VOLCANOES OF OLDER DATE. 
 
 The district lying to the west of Naples, in which 
 the Monte Nuovo is situated, contains a great number 
 of hills, all of which present a most striking similarity 
 to that volcano. All these hills are truncated cones, 
 with larger or smaller circular depressions at their sum- 
 mits, and they are entirely composed of volcanic scoriae, 
 lapilli, and dust. Some of these hills are of considerably 
 larger dimensions than the Monte Nuovo, while others 
 
OLDER VOLCANOES OF THE CAMPi-PStfi&KJEI. 79 
 
 are of smaller size, as shown in the annexed map, 
 fig. 11. No stranger visiting the district, without 
 previous information upon the subject, would ever 
 suspect the fact that, while all the other hills of the 
 district have existed from time immemorial, and are 
 constantly mentioned in the works of Greek and Ro- 
 man writers, this particular hill of Monte Nuovo came 
 into existence less than 350 years ago. 
 
 The evidently fused condition of the materials of 
 which these hills are built up is a clear sign of the 
 volcanic action which has taken place in it ; and this 
 fact was so fully recognised by the ancients that they 
 called the district the Campi Phlegraei, or ' the Burn- 
 ing Fields,' and regarded one of the circular depres- 
 sions in it as the entrance to Hades. 
 
 It is impossible for anyone to examine this district 
 without being convinced that all the numerous cones 
 and craters which cover it have been formed by the 
 same agency as that by which Monte Nuovo was pro- 
 duced. We have shown that there is the most satis- 
 factory historical evidence as to what that agency was. 
 
 Now volcanic cones with craters in their centres 
 occur in great numbers in many parts of the earth's 
 surface. In some districts, like the Auvergne, the 
 Catacecaumene in Asia Minor, and certain parts of 
 New Zealand, these volcanic cones occur by hundreds 
 and thousands. In some instances, these volcanic cones 
 have been formed in historic times, but in the great 
 majority of cases we can only infer their mode of origin 
 
80 VOLCANOES. 
 
 from their similarity to others of which the formation 
 has been witnessed. 
 
 Most of the smaller volcanic hills, with their 
 craters, have been thrown up during a single eruption 
 from a volcanic fissure ; but, as Hamilton conclusively 
 proved, the grandest volcanic mountains must have 
 been produced by frequent repetitions of similar oper- 
 ations upon the same site. For not only are these 
 great volcanic piles found to be entirely composed of 
 materials which have evidently been ejected from 
 volcanic vents, but, when carefully watched, such 
 mountains are found undergoing continual changes in 
 form, by the addition of materials thrown out from the 
 vent, and falling upon their sides. 
 
 This fact will be well illustrated by a comparison 
 of the series of drawings of the summit of Vesuvius 
 which were made by Sir William Hamilton in 1767, 
 and which we have copied in fig. 12. During the earlier 
 months of that year the summit of the mountain was 
 seen to be of truncated form, a great crater having 
 been originated by the violent outbursts of the pre- 
 ceding year. This condition of the mountain-top is 
 represented in the first figure of the series. The 
 drawing made by Hamilton, on July 8, shows that not 
 only was the outer rim of the great crater being 
 modified in form by the fall of materials upon it, but 
 that in the centre of the crater a small cone was being 
 gradually built up by the quiet ejections which were 
 taking place. If we compare the drawings made at 
 
FIG. 12. OUTLINES OF THE SUMMIT OF VESUVIUS DURING THE 
 ERUPTION OF 1767. 
 
CHANGES IN FORM OF VESUVIUS. 81 
 
 successive dates, we shall find that the constant showers 
 of falling materials were not only raising the edge of the 
 great crater but were at the same time increasing the 
 size of the small cone inside the crater. By the end of 
 October the small cone had grown to such an extent 
 that its sides were confluent with those of the principal 
 cone, which had thus entirely lost its truncated form 
 and been raised to a much greater height. The com- 
 parison of these drawings will be facilitated by the 
 dotted lines, which represent the outline of the top of 
 the mountain at the preceding observation ; so that the 
 space between the dotted and the continuous line in 
 each drawing shows the extent to which the bulk of 
 the cone had increased in the interval between two 
 observations. 
 
 But, although the general tendency of the action 
 going on at volcanic mountains is to increase their 
 height and bulk by the materials falling upon their 
 summits and sides, it must be remembered that this 
 action does not take place by any means continuously 
 and regularly. Not only are there periods of rest in 
 the activity of the volcano, during which the rain 
 and winds may accomplish a great deal in the way of 
 crumbling down the loose materials of which volcanic 
 mountains are largely built up, but sudden and violent 
 eruptions may in a very short time undo the slow work 
 of years by blowing away the whole summit of the 
 mountain at once. Thus, before the great eruption of 
 1822, the cone of Vesuvius, by the almost constant 
 5 
 
82 
 
 VOLCANOES. 
 
 ejection of ashes during several years, had been raised 
 to the height of more than 4,000 feet above the level of 
 the sea ; but by the terrible outburst 'which then took 
 place the cone was reduced in height by 400 feet, and 
 a vast crater, which had a diameter of nearly a mile, 
 and a depth of nearly 1,000 feet (see fig. 13), was 
 formed at the top of the mountain. The enormous 
 quantity of material thus removed was either distributed 
 
 FIG. 13. CRATER OF VESUVIUS FORMED DURING THE ERUPTION IN 
 1822. (It was nearly 1 mile in diameter and 1,000 ft. deep.) 
 
 over the flanks of the mountain, or, when reduced to a 
 finely comminuted condition, was carried by the wind 
 to the distance of many miles, darkening the air, and 
 coating the surface of the ground with a thick covering 
 of dust. 
 
 The volcano of Vesuvius, although of somewhat 
 insignificant dimensions when compared with the 
 grander volcanic mountains of the globe, possesses 
 
EARLY HISTOKY OF VESUVIUS. 83 
 
 great interest for the student of Vulcanology, inas- 
 much as being situated in the midst of a thickly 
 populated district and in close proximity to the city 
 of Naples, it has attracted much attention during past 
 times, and there is no other volcano concerning which 
 we have so complete a series of historical records. 
 The present cone of Vesuvius, which rises within the 
 great encircling crater-ring of Somma, has a height of 
 about 1.3,000 feet. But there is undoubted evidence 
 that this cone, to the top of which a railway has re- 
 cently been constructed for the convenience of tourists, 
 has been entirely built up during the last 1,800 years, 
 and, what is more, that during this period it has been 
 many times almost wholly destroyed and reconstructed. 
 Nothing is more certain than the fact that the 
 Vesuvius upon which the ancient Romans and the 
 Greek settlers of Southern Italy looked, was a moun- 
 tain differing entirely in its form and appearance from 
 that with which we are familiar. The Vesuvius known 
 to the ancients was a great truncated cone, having a 
 diameter at its base of eight or nine miles, and a 
 height of about 4,000 feet. The summit of this moun- 
 tain was formed by a circular depressed plain, nearly 
 three miles in diameter, within which the gladiator 
 Spartacus, with his followers, were besieged by a Roman 
 army. There is no evidence that at this time the vol- 
 canic character of the mountain was generally recog- 
 nised, and its slopes are described by the ancient 
 geographers as being clothed with fertile fields and 
 
84 
 
 VOLCANOES. 
 
 vineyards, while the hollow at the top was a waste 
 overgrown with wild vines, 
 
 But in the year 79 a terrible and unexpected erup- 
 tion occurred, by which a vast, crateral hollow was 
 formed in the midst of Vesuvius, and all the southern 
 side of the great rim surrounding this crater was 
 broken down. Under the materials ejected during this 
 
 FIG. 14. CRATER OF VESUVIUS ix 1756. 
 (From a drawing made on the spot.) 
 
 eruption, the cities of Pompeii, Herculaneum, and 
 Stabile were overwhelmed and buried. 
 
 Numerous descriptions and drawings enable us to 
 understand how in the midst of the vast crater formed 
 in the year 79 the modern cone has gradually been 
 built up. Fresh eruptions are continually increasing 
 the bulk, or raising the height of the Vesuvian cone. 
 
 The accompanying drawings made by Sir William 
 Hamilton enable us to understand the nature of 
 
VESUVIUS IN MODERN TIMES. 
 
 85 
 
 the changes which have been continually taking place 
 at the summit of Vesuvius. The drawing fig. 14 
 shows the appearance presented by the crater in the 
 year 1756. 
 
 At this time we see that inside the crater a series 
 of cones had been built up one within the other from 
 which lava issued, filling the bottom of the crater and 
 
 FIG. 15. THE SUMMIT OF VESUVIUS IN 1767. 
 (From an original drawing.) 
 
 finding its way through a breach in its walls, down the 
 side of the cone. It is evident that the ejected mate- 
 rials falling on the sides of the innermost cone would 
 tend to enlarge the latter till its sides became confluent 
 with the cone surrounding it, and if this action went 
 on long enough, the crater would be entirely filled up 
 and a perfect cone with only a small aperture at the 
 top would be produced. But from time to time, grand 
 and paroxysmal outbursts have occurred at Vesuvius, 
 
86 
 
 VOLCANOES. 
 
 which have truncated the cone, and sometimes formed 
 great, cup-shaped cavities, reaching almost to its base, 
 like that shown in fig. 13. 
 
 In 1767 the crater of Vesuvius, as shown in fig. 
 15, contained a single small cone in a state of constant 
 spasmodic outburst, like that of Stromboli. 
 
 if if ,/M(|! 
 
 Fm. 16. SUMMIT OF VESUVIUS IN 1843. 
 
 In 1843, we find that the crater of Vesuvius con- 
 tained three such small cones arranged in a line along 
 its bottom as depicted in fig. 16. 
 
 These drawings of the summit of Vesuvius give a 
 fair notion of the changes which have been continually 
 going on there during the whole of the historical period. 
 Ever and anon a grand outburst, like that of 1822, has 
 produced a vast and deep crater such as is represented 
 in fig. 13, and then a long continuance of quiet and 
 
CHANGES IN OUTLINE OF VESUVIUS. 
 
 87 
 
 
 regular ejections has built up within the 
 cones like those shown in 
 figs. 14, 15 and 16, till at 
 last the great crater has been 
 completely filled up, and the 
 cone reconstructed. 
 
 In the series of outlines 
 in fig. 17, we have endea- 
 voured to illustrate the suc- 
 cession of changes which 
 has taken place in Vesu- 
 vius during historical times. 
 In the year 79 one side 
 of the crater-wall of the 
 vast mountain - mass was 
 blown away. Subsequent 
 ejections built up the present 
 cone of Vesuvius within the 
 great encircling crater-wall 
 of Somma, and the form of 
 this cone and the crater at 
 its summit have been un- 
 dergoing continual changes 
 during the successive erup- 
 tions of eighteen centuries. 
 
 What its future history 
 may be we can only conjec- 
 ture from analogy. It may 
 be that a long continuance 
 
 crater small 
 
88 VOLCANOES. 
 
 of eruptions of moderate energy may gradually raise the 
 central cone till its sides are confluent with those of 
 the original mountain ; or it may be that some violent 
 paroxysm will entirely destroy the modern cone, re- 
 ducing the mountain to the condition in which it was 
 after the great outburst of 79. On the other hand, 
 if the volcanic forces under Vesuvius are gradually 
 becoming extinct (but of this we have certainly no 
 evidence at present), the mountain may gradually sink 
 into a state of quiescence, retaining its existing form. 
 The series of changes in the shape of Vesuvius, 
 which are proved by documentary evidence to have 
 been going on during the last 2,000 years, probably 
 find their parallel in all active volcanoes. In all of 
 these, as we shall hereafter show, the activity of the 
 vents undergoes great vicissitudes. Periods of con- 
 tinuous moderate activity alternate with short and 
 violent paroxysmal outbursts and intervals of complete 
 rest, which may in some cases last for hundreds or 
 even thousands of years. During the periods of con- 
 tinuous moderate activity, the crater of the volcano is 
 slowly filled up by the growth of smaller cones within 
 it ; and the height of the mountain is raised. By the 
 terrible paroxysmal outbursts the mountain is often 
 completely gutted and its summit blown away ; but 
 the materials thus removed from the top and centre of 
 the mass are for the most part spread over its sides, 
 so that its bulk and the area of its base are thereby 
 increased. During the intervals of rest, the sides of 
 
DEVIATIONS FROM CONICAL FORM. 89 
 
 the mountain which are so largely composed of loose 
 and pulverulent materials are washed downwards by 
 rains and driven about by winds. Thus all volcanoes ( 
 in a state of activity are continually growing in size ; 
 every ejection, except in the case of those where the 
 materials are in the finest state of subdivision, adding 
 to their bulk ; the area of their bases being increased 
 during paroxysmal outbursts, and their height during 
 long-continued moderate eruptions. 
 
 We have pointed out that the conical form of vol- 
 canic mountains is due to the slipping of the falling 
 materials over one another till they attain the angle 
 at which they can rest. There are, however, some 
 deviations from this regular conical form of volcanoes 
 which it may be well to refer to. 
 
 The quantity of rain which falls during volcanic 
 eruptions is often enormous, owing to the condensation 
 of the great volumes of steam emitted from the vent. 
 Consequently the falling lapilli and dust often descend 
 upon the mountain, not in a dry state but in the con- 
 dition of a muddy paste. Many volcanic mountains 
 have evidently been built up by the flow of successive 
 masses of such muddy paste over their surfaces. Some 
 volcanic materials when mixed with water -have the 
 property of rapidly < setting ' like concrete. The 
 ancient Romans and modern Italians, well acquainted 
 with this property of certain kinds of volcanic dust and 
 lapilli, have in all ages employed this 4 puzzolana,' as it 
 is called, as mortar for building. The volcanic muds 
 
90 VOLCANOES. 
 
 have often set in their natural positions, so as to form 
 a rock, which, though light and porous, is of tolerably 
 firm consistency. To this kind of rock, of which 
 Naples and many other cities are built, the name of 
 6 tuff' or c tufa ' is applied. A similar material is known 
 in Northern Grermany as * trass.' 
 
 The cause of the ' setting ' of puzzolana and tufa 
 is that rain-water containing a small proportion of car- 
 bonic acid acts on the lime in the volcanic fragments, 
 and these become cemented together by the carbonate 
 of lime and the free silica, which are thus produced in 
 the mass. 
 
 When a strong wind is blowing during a volcanic 
 outburst, the materials may be driven to one side of 
 the vent, and accumulate there more rapidly than on 
 the other. Thus lop-sided cones are formed, such as 
 may frequently be observed in some volcanic districts. 
 In areas where constant currents of air, like the trade- 
 winds, prevail, all the scoria-cones of the district may 
 thus be found to be unequally developed on opposite 
 sides, being lowest on those from which the prevalent 
 winds blow, and highest on the sides towards which 
 these winds blow. 
 
 The examination of any careful drawing, or better 
 still of the photograph, of a volcanic cone, will prove 
 that the profile of such cones is not formed by straight 
 lines, but by curves often of a delicate and beautiful 
 character. The delineations of the sacred volcano of 
 Fusiyama, which are so constantly found in the pro- 
 
ANGLE OF SLOPE IN VOLCANIC CONES. 91 
 
 ductions of Japanese artists, must have familiarised 
 everyone with the elegant curved lines exhibited by 
 the profiles of volcanoes. The upper slope of the 
 mountain is comparatively steep, often exhibiting 
 angles of 30 to 35, but this steepness of slope gra- 
 dually diminishes, till it eventually merges in the 
 surrounding plains. The cause of this elegant form 
 assumed by most volcanic mountains is probably two- 
 fold. In the first place we have to remember that the 
 materials falling upon the flanks of the mountain 
 differ in size and shape, and some will rest on a steeper 
 slope than others. Thus, while some of the materials 
 remain on the upper part of the mountains, others are 
 rolling outwards and downwards. Hence we find that 
 those cones which are composed of uniform materials 
 have straight sides. But in some cases, we shall see 
 hereafter, there has certainly been a central subsidence 
 of the mountain mass, and it is this subsidence which 
 has probably given rise to the curvature of its flanks. 
 
 We have hitherto considered only the methods by 
 which the froth or foam, which accumulates on the 
 surface of fluid lava, is dispersed. But in many cases 
 not only is this scum of the lava ejected from the 
 volcanic vent by the escaping steam, but the fluid lava 
 itself is extruded forcibly, and often in enormous 
 quantities. 
 
 The lava in a volcanic vent is always in a highly 
 heated, usually incandescent, condition. Seen by night, 
 its freshly exposed surface is glowing red, sometimes 
 
92 VOLCANOES. 
 
 apparently white-hot. But by exposure to the atmo- 
 sphere the surface is rapidly chilled, appearing dull red 
 by night, and black by day. Many persons are surprised 
 to find that a flowing stream of lava presents the ap- 
 pearance of a great mass of rough cinders, rolling along 
 with a rattling sound, owing to the striking of the clin- 
 ker-like fragments against each other. When viewed 
 by night, the gleaming, red light between these rough, 
 cindery masses betrays the presence of incandescent 
 materials below the chilled surface of the lava-stream. 
 
 No fact in connection with lavas is more striking 
 than the varying degrees of liquidity presented by 
 them in different cases. While some lava-streams 
 seem to resemble rivers, the material flowing rapidly 
 along, filling every channel in its course, and deluging 
 the whole country around, others would be more fitly 
 compared to glaciers, creeping along at so slow a rate 
 that the fact of their movement can only be demon- 
 strated by the most careful observation. Even when 
 falling over a precipice such lavas, owing to their im- 
 perfect liquidity, form heavy, pendent masses like a 
 'guttering' candle, as is shown by fig. 18, which is 
 taken from a drawing kindly furnished to me by Capt. 
 S. P. Oliver, R.N. The causes of these differences in 
 the rate of motion of lava-streams we must proceed to 
 consider. 
 
 There can be no doubt that the temperature of 
 lavas varies greatly in different cases. This is shown 
 by the fact that while some lavas are in a state of com- 
 
TEMPERATURE OF LAVA-STREAMS. 
 
 93 
 
 plete fusion, similar to that of the slags of furnaces, 
 and like the latter, such lavas on cooling form a glassy 
 mass, others consist of a liquid magma in which a larger 
 or smaller number of crystals are found floating. In 
 these latter cases the temperature of the magma must 
 be below the fusing-point of the minerals which exist 
 
 FIG, 18. CASCADE OF LAVA TUMBLING OVER A CLIFF ix THE ISLAND 
 OF BOURBON. 
 
 in a crystalline condition in its midst. It has indeed 
 been suggested that the whole of the crystals in lavas 
 are formed during the cooling down of a completely 
 fused mass ; but no one can imagine that the enclosed 
 crystals of quartz, felspar, leucite, olivine, &c., have 
 been so formed, such crystals being sometimes more 
 than an inch in diameter. The microscopic examina- 
 
94 VOLCANOES. 
 
 tion of lavas usually enables us to discriminate between 
 those complete crystals which have been formed at 
 great depths and floated to the surface, and the minute 
 crystalline particles and microliths which have been 
 developed in the glassy mass during cooling. Crystals 
 of the former class, indeed, exhibit abundant evidence, 
 in their liquid cavities and other peculiarities, that they 
 have not been formed by simple cooling from a state 
 of fusion, but under the combined action of heat, 
 the presence of water and various gases, and intense 
 pressure. 
 
 As we have already seen, the different lavas vary 
 greatly in their degrees of fusibility. The basic lavas, 
 containing a low percentage of silica, are much more 
 fusible than the acid lavas, which contain a high per- 
 centage of silica.. When the basic lavas are reduced to 
 a complete state of fusion their liquidity is sometimes 
 very perfect, as is the case at Kilauea in Hawaii, where 
 the lava is thrown up into jets and fountains, falling in 
 minute drops, and being drawn out into fine glassy 
 threads. On the other hand, the less fusible acid lavas 
 appear to be usually only reduced to the viscous or 
 pasty condition, which artificial glasses assume long 
 before their complete fusion. Of this fact I have found 
 many proofs in the Lipari Islands, where such glassy, 
 acid lavas abound. In fig. 6 (page 43) a lava-stream 
 is represented on the side of the cone of Vul- 
 cano. This lava is an obsidian that is to say, it is of 
 the acid type and completely glassy but its liquidity 
 
IMPERFECTLY FLUID LAVAS. 
 
 95 
 
 must have been very imperfect, seeing that the stream 
 has come to a standstill before reaching the bottom of 
 a steep slope of about 35. In fig. 1 9 there is given a 
 side view of the same stream of obsidian, from which 
 it will be seen that it has flowed slowly down a steep 
 slope and heaped itself up at the bottom, as its fluidity 
 
 FIG. 19. LAVA-STREAM (OBSIDIAN) IN THE ISLAND OF VULCANO 
 SHOWING THE IMPERFECT LIQUIDITY OF THE MASS. 
 
 was not complete enough to enable it to move on a 
 slighter incline. An examination of the interior of 
 such imperfectly fluid lavas affords fresh proofs of the 
 slow and tortuous movements of the mass. Everywhere 
 we find that the bands of crystallites and sphaerulites 
 are, by the movement of the mass, folded and crumpled 
 and puckered in the most remarkable manner, as is 
 illustrated in figs. 20 and 21. Similar appearances 
 
VOLCANOES. 
 
 occur again and again among the vitreous and semi- 
 vitreous acid lavas of Hungary. 
 
 FIG. 20. INTERIOR OF A RHYOLITIC LAVA-STREAM IN THE ISLAND OF 
 LlPARI, SHOWING BROAD SIGMOIDAL FOLDS PRODUCED BY THE SLOW 
 MOVEMENTS OF THE MASS. 
 
 FIG. 21. INTERIOR OF A RHYOLITIC LAVA-STREAM IN THE ISLAND OF 
 LlPARI, SHOWING THE COMPLICATED CRUMPLINGS AND PUCKERINGS 
 PRODUCED BY THE SLOW MOVEMENTS OF THE MASS. 
 
 But, although the temperature of lava-streams and 
 the fusibility of their materials may in 'some cases 
 
KATE OF MOVEMENT OF VESUVIAN LAVAS. 97 
 
 account for their condition of either perfect liquidity 
 or viscidity, it is clear that in other instances there 
 must be some other cause for this difference. Thus it 
 has been found that at Vesuvius the lavas erupted in 
 modern times have all a striking similarity to one 
 another in chemical composition, in the minerals which 
 they contain, and in their structure. They are all basic 
 lavas, which when examined by the microscope are 
 seen to consist of a more or less glassy magma, in the 
 midst of which numerous crystals of augite, leucite, 
 olivine, magnetite, and other minerals are scattered 
 Yet nothing can be more strikingly different than 
 the behaviour of the lavas poured out from Vesuvius at 
 various periods. In some cases the lava appears to be 
 in such a perfectly liquid condition that, issuing from 
 the crater, it has been described as rushing down the 
 slope of the cone like a stream of water, and such ex- 
 ceedingly liquid lavas have in some cases flowed to the 
 distance of several miles from the base of the moun- 
 tain in a very short time. But other Vesuvian lavas 
 have been in such a viscid condition that their rate of 
 movement has been so extremely slow as to be almost 
 imperceptible. Such lava-streams have continued in 
 movement during many years, but the progress has 
 been so slow (often only a few inches in a day) that 
 it could only be proved by means of careful measure- 
 ments. 
 
 If we examine some of these Vesuvian lavas which 
 have exhibited such striking differences in their rate of 
 
98 VOLCANOES. 
 
 flow, we shall find that they present equally marked 
 differences in the character of their surfaces. The lava- 
 current of 1858 was a remarkable example of a slow- 
 flowing stream, and its surface, as will be seen in fig. 22, 
 which is taken from a photograph, has a very marked 
 and peculiar character. A tenacious crust seems to have 
 formed on the surface, and by the further motion of 
 the mass this crust or scum has been wrinkled and 
 folded in a very remarkable manner. Sometimes this 
 folded and twisted crust presents a striking resemblance 
 to coils of rope. Precisely similar appearances may be 
 observed on the surface of many artificial slags when 
 they flow from furnaces, and are seen to be due to the 
 same cause, namely, the wrinkling up of the chilled 
 surface-crust by the movement of the liquid mass below. 
 Lavas which prc-sent this appearance are frequently called 
 ' ropy lavas ' ; an admirable example of them is afforded 
 in the lava-cascade of the Island of Bourbon represented 
 in fig. 18 (page 93). 
 
 But lavas in w^hich the rate of flow has been very 
 rapid, exhibit quite a different kind of surface to 
 that of the ropy lavas. The Vesuvian lava-stream 
 of 1872 was remarkable for the rapidity of its flow, 
 and its surface presents a remarkable contrast to that 
 of the slow-moving lava of 1858. The surface of the 
 lava-current of 1872 is covered with rough cindery 
 masses, often of enormous dimensions, and it is exceed- 
 ingly difficult to traverse it, as the ragged projecting 
 fragments tear the boots and lacerate the skin. The 
 
FIG. 22. YESUTIAX LAVA-STREAM OP 1858, 
 
 EXHIBITING THE PECULIAR ' ROPY ' SUBFACES OF SLOWLY 
 MOVTS'G CURRENTS, 
 
 (From a Photograph,) 
 
FIG. 23. VESUVIAN LAVA-STEEAM OF 1872, 
 
 EXHIBITING THE BOUGH ClNDERY SURFACES CHARACTERISTIC OF 
 
 KAPIDLY FLOWING CURRENTS. 
 
 (From a Photograph.} 
 
VESUVIAN LAVA-STKEAM OF 1872. 99 
 
 appearance presented by this lava- stream is illustrated 
 by fig. 23, which is also taken from a photograph. 
 
 Now it is found that those lava-streams which move 
 slowly and present ropy surfaces give off but little 
 steam during their flow, while those lava-streams which '. 
 flow more rapidly and present a rough and cindery. 
 appearance give off vast quantities of steam. The 
 extraordinary amount of vapour given off from the lava- 
 streams which flowed from Vesuvius in 1872 is illus- 
 trated in the photograph copied in fig. 5 (facing page 24), 
 in which the three lava-currents are each seen to be 
 surmounted by enormous vapour-clouds rising to the 
 height of several thousands of feet above them, and 
 mingling with the column that issued from the central 
 vent. By the escape of this enormous quantity of steam 
 the surface of the lava was thrown into rugged cindery 
 projections, and in some places little cones were formed 
 upon it, which threw out small scoriae and dust. The 
 quantity of vapour was, in fact, so great, that little 
 parasitical volcanoes were formed on the surface of the 
 lava-stream. Some of these miniature volcanoes were 
 of such small dimensions that they were carried away 
 on boards to be employed as illustrations in the lecture- 
 rooms of the University of Naples. 
 
 The arrangement of the materials forced out from 
 fissures on the surfaces of lava-streams by the disen- 
 gaged vapours and gases depends on the degree of 
 fluidity of the lava, and the force of the escaping steam- 
 jets. In very viscous lavas the materials may issue 
 
100 
 
 VOLCANOES. 
 
 quietly, forming great concentric masses like coils of 
 rope ; such were described by Mr. Heaphy as occurring 
 in New Zealand (see fig. 24). 
 
 FIG. 24. CONCENTRIC FOLDS ON MASS OF COOLED LAVA. 
 
 In other cases the lava, if somewhat more liquid, may 
 in issuing quietly without great outbursts of steam, 
 accumulate in great bottle-shaped masses, which have 
 
 FIG. 25. MASS OF COOLED LAVA FORMED OVER A SPIRACLE ON THE 
 SLOPES OF HAWAII. 
 
 been compared to 6 petrified fountains.' Cases of this 
 kind have been described by Professor Dana as occur- 
 ring on the slopes of Hawaii (see fig. 25). 
 
 When the steam escapes with explosive violence 
 
MINIATURE CONES ON LAV A- STREAMS. 101 
 
 from a spiracle (' bocca ') on the surface of a lava-stream, 
 minute cinder cones, like those described as being 
 formed in 1872, are the result. Fig. 26 represents a 
 group of miniature cones thrown up on the Vesuvian 
 lava-stream of 1855 : it is taken from a drawing by 
 Schmidt. 
 
 Some of these appear like burst blisters or bubbles, 
 while others are built up of scoriaceous masses which 
 
 FIG. 26. GROUP OF SMALL CONES THROWN UP ON THE VESUVIAN 
 LAVA-CURRENT OF 1855. 
 
 have been ejected from the aperture and have become 
 united while in a semi-fluid condition. Other examples 
 of these spiracles or bocche on the surfaces of lava-cur- 
 rents may be seen in the figs. 22 and 23, which are 
 copied from photographs. 
 
 The facts we have described all point to the con- 
 clusion that the presence of large quantities of water 
 imprisoned in a mass of lava contributes greatly to its 
 mobility. And this conclusion is supported by so many 
 
102 VOLCANOES 
 
 other considerations that it is now very generally ac- 
 cepted by geologists. The condition of this imprisoned 
 water in lavas is one which demands further investi- 
 gation at the hands of physicists. It has been sug- 
 gested, with some show of reason, that the water may 
 exist in the midst of the red-hot lava as minute parti- 
 cles in the curious ' spheroidal condition ' of Boutigny, 
 and that these flash into steam as the lava flows along. 
 
 Lava, when extruded from a volcanic crater in a 
 more or less completely fluid state, flows down the side 
 of the cone, and then finds its way along any channel 
 or valley that may lie in its course, obeying in its 
 movements all the laws of fluid bodies. The lava- 
 currents thus formed are sometimes of enormous 
 dimensions, and may flood the whole country for many 
 miles around the vent. 
 
 Lava-streams have been described, which have 
 flowed for a distance of from fifty to a hundred miles 
 from their source, and which have had a breadth vary- 
 ing from ten to twenty miles. Some lava-streams 
 have a thickness of 500 feet, or even more. These 
 measures will give some idea of the enormous quanti- 
 ties of material brought from the earth's interior by 
 volcanic action and distributed over its surface. The 
 mass of lava which flowed out during an eruption off 
 Reykjanes in Iceland, in the year 1783, has been calcu- 
 lated to be equal in bulk to Mont Blanc. 
 
 There are many parts of the earth's surface, such 
 as the Western Isles of Scotland and the North-east of 
 
FEATURES OF LAVA-STREAMS. 103 
 
 Ireland, the Deccan of India, and large tracts in the 
 Rocky Mountains, where successive lava-sheets have 
 been piled upon one another to the height of several 
 thousands of feet, and cover areas of many hundreds or 
 even thousands of square miles. 
 
 The more fusible basic lavas are as a general rule 
 more liquid in character than any others, and it is these 
 very liquid lavas that are usually found forming plateaux 
 built up of successive lava-streams. The less liquid 
 lavas, like those of Hungary and Bohemia, are not 
 usually found flowing to such distances from the vent, 
 but form dome-shaped mountain-masses. 
 
 Lava-streams usually exhibit in their upper and 
 under surfaces a scoriaceous texture due to the escape 
 of steam from the upper surface, portions of the cindery 
 masses so formed falling off from the end of the stream, 
 and being rolled over by the stream so as to form its 
 base. The thickness of this scoriaceous upper and 
 lower part of a lava-stream varies according to the 
 quantity of steam imprisoned in it ; but all thick lava- 
 streams have a compact central portion which is com- 
 posed of hard, solid rock. Very good examples of 
 the internal structure of lava-streams may sometimes 
 be examined in the sea-cliffs of volcanic islands. In 
 fig. 27 we have given a copy of a drawing made while 
 sailing round the shores of Vulcano. The scoriaceous 
 portions of lava-streams are sometimes employed, as at 
 Volvic in the Auvergne, as a building material, or as at 
 Neidermendig in the Eifel, and in Hungary for mill- 
 
104 
 
 VOLCANOES. 
 
 stones ; the compact portions are employed for building 
 and paving, and for road metal. The rock of some of 
 the modern lava-streams of Vesuvius is largely quarried 
 for paving the streets of Naples. 
 
 This solid portion of the lava-streams in slowly 
 cooling down from its highly-heated condition under- 
 goes contraction, and in consequence is rent asunder 
 by a number of cracks. Sometimes these cracks 
 
 FIG. 27. NATURAL SECTION OF A LAVA-STREAM IN THE ISLAND OF 
 VULCANO, SHOWING THE COMPACT CENTRAL PORTION AND THE 
 SCORIACEOUS UPPER AND UNDER SURFACES. 
 
 assume a wonderfully regular arrangement, and the 
 rock may be broken up into very symmetrical masses. 
 
 If we imagine a great sheet of heated material, 
 like a lava-stream, slowly cooling down, it is evident that 
 the contraction which must take place in it will tend 
 to produce fissures breaking up the mass into prisms. 
 A little consideration will convince us what the form of 
 these prisms must be. There are only three regular 
 figures into which a surface can be divided, namely, 
 equilateral triangles, squares, and regular hexagons; 
 the first being produced by the intersection of sets of 
 six lines radiating at angles of 60 from certain centres ; 
 
COLUMNAR STRUCTURE OF LAVAS. 105 
 
 the second by the intersection of sets of four lines 
 radiating from centres at angles of 90 ; and the third 
 from sets of three lines radiating from centres at an ' 
 angle of 120. It is evident that a less amount of 
 contractile force will be required to produce the sets 
 of three cracks rather than those of four or six cracks ; 
 or, in other words, the contractile force in a mass will 
 be competent to produce the cracks which give rise to 
 hexagons rather than those which form squares or tri- 
 angles. This is no doubt the reason why the prisms 
 formed by the cooling of lava, as well as those produced 
 during the drying of starch or clay, are hexagonal in 
 form. 
 
 The hexagonal prisms or columns formed by con- 
 traction during the consolidation of lavas vary greatly 
 in size, according to the rate of cooling, the nature of 
 the materials, and the conditions affecting the mass. 
 Sometimes such columns may be found having a 
 diameter of eight or ten feet and a length of five hun- 
 dred feet, as in the Shiant Isles lying to the north of 
 the Island of Skye ; in other cases, as in certain volcanic 
 glasses, minute columns, an inch or two in length and 
 scarcely thicker than a needle, are formed ; and exam- 
 ples of almost every intermediate grade between these 
 two extremes may sometimes be found. The largest 
 columns are those which are formed in very slowly 
 cooling masses. 
 
 The columnar structure is exnibited by all kinds of 
 lava, and indeed in other rock-masses which have been 
 6 
 
106 
 
 VOLCANOES. 
 
 heated by contact with igneous masses and gradually 
 cooled. The rocks which display the structure in 
 greatest perfection, however, are the basalts. 
 
 Mr. Scrope first called attention to the fact that the 
 upper and lower portions of lava-streams sometimes 
 cool in very different ways, and hence produce columns 
 of dissimilar character. The lower portion of the mass 
 parts with its heat very slowly, by conduction to the 
 
 FIG. 28. SECTION OF A LAVA-STHKAM EXPOSED ON THE SIDE OF THE 
 UIVER ARDECHE, IN THE SOUTH WEST OF FRANCE. 
 
 underlying rocks, while the upper portions radiate heat 
 more irregularly into the surrounding atmosphere. 
 Hence we often find the lower portions of thick lava- 
 streams to be formed of stout, vertical columns of great 
 regularity ; while the upper part is made up of smaller 
 and less regular columns, as shown in fig. 28. 
 
 The remarkable grotto known as Fingal's Cave in 
 the Island of Staffa has been formed in the midst of a 
 lava-stream such as we have been describing ; the 
 thick vertical columns, which rise from beneath the 
 level of the sea, are divided by joints and have been 
 
OTHER JOINT-STRUCTUKES IN LAVAS. 107 
 
 broken away by the action of the sea ; in this way a 
 great cavern has been produced, the sides of which are 
 formed by vertical columns, while the roof is made up 
 of smaller and interlacing ones. The whole structure 
 bears some resemblance to a Gothic cathedral ; the sea 
 finding access to its floor of broken columns, and per- 
 mitting the entrance of a boat during fine weather. 
 Similar, though perhaps less striking, structures are 
 found in many other parts of the globe wherever basaltic 
 and other lava-streams exhibit the remarkable columnar 
 structure as the result of their slow cooling. Portions of 
 basaltic columns are often employed for posts by the 
 road-sides, as in Central Germany and Bohemia, or for 
 paving stones, as in Pompeii and at the Monte Albano 
 near Kome. 
 
 Occasionally basaltic lava-streams exhibit other cu- 
 rious structures in addition to the columnar. Thus 
 some basaltic columns are 
 found divided into regular 
 joints by equidistant,curved 
 surfaces, the joints thus 
 fitting into one another by 
 a kind of ball-and-socket 
 arrangement. Sometimes 
 we find processes project- 
 
 FIG. 29. PORTION OF A BASALTIC 
 ing from the angles of the COLUMN FROM THE GIANT'S CAUSE- 
 
 . WAY, EXHIBITING BOTH THE BALL- 
 
 CUrvedjOint-SUrfaceSo which AND-SOCKET AND THE TENON-AND- 
 
 MOKTISE STRUCTURES. 
 
 cause the blocks to fit to- 
 gether as with a tenon and mortise. This kind of 
 
108 
 
 VOLCANOES. 
 
 structure is admirably displayed at the Giant's Cause- 
 way, Co. Antrim, in the North of Ireland. A por- 
 tion of a basaltic column from this locality is repre- 
 sented in fig. 29. 
 
 While the ordinary columnar structures are very 
 common in basalts, the ball-and-socket and tenon-and- 
 
 FIG. 30. VEIN OF GREEX PITCHSTONE, AT CHIAJA DL LUNA IN THE 
 ISLAND OF PONZA, BREAKING UP INTO REGULAR COLUMNS, AND INTO 
 SPHERICAL MASSES WITH A CONCENTRIC SERIES OF JOINTS. 
 
 mortise structures are exceedingly rare. The question 
 of the mode of origin of these remarkable struc- 
 tures has given rise to much discussion, and the 
 opinions of geologists and physicists are by no means 
 unanimous upon the subject. 
 
 Sometimes we find masses of lava traversed by curved 
 
PERLITIC- STRUCTURE IN LAVAS. 109 
 
 joints, and occasionally we find curious combinations 
 of curved and plane joints, giving rise to appearances 
 scarcely less remarkable than those presented by the 
 columns of the Griant's Causeway. Some of the more 
 striking examples of this kind have been described and 
 explained by Professor Bonney. 
 
 In the Ponza Islands there occurs a remarkable 
 
 FIG. 31. ILLUSTRATION OF THK 'PEULITIC STRUCTURE'. IN GLASSY ROCKS. 
 
 a. Perlitic structure, as seen in a lava from Hungary. 
 
 6. The same structure, artificially produced in Canada balsam during cooling. 
 
 example of a columnar pitch stone, which is also traversed 
 by a number of curved concentric joints, causing the 
 rock to break up into pieces like the coats of an onion. 
 This remarkable rock-mass is represented in fig. 30. 
 
 A very similar structure is often seen in certain 
 glassy lavas, when they are examined in thin sections 
 under the microscope. Such glassy lavas exhibit the 
 
110 VOLCANOES. 
 
 peculiar lustre of mother-of-pearl, doubtless in conse- 
 quence of the interference of light along the cracks. 
 Lavas exhibiting this character are known to geologists 
 as ' perlites.' The perlitic structure has been produced 
 artificially by Mr. Grenville Cole in Canada, Balsam, and 
 by MM. Fouque and Michel Levy, in chemically 
 deposited silica. See fig. 31. 
 
 A thick lava-stream must take an enormous period 
 to cool down probably many hundreds or even thou- 
 sands of years. It is possible to walk over lava-streams 
 in which at a few inches below the surface the rock is 
 still red-hot, so that a piece of stick is lighted if 
 thrust into a crack. Lava is a very bad conductor of 
 heat, and loose scoriae and dust are still worse con- 
 ductors. During the eruption of Vesuvius in 1872, 
 masses of snow which were covered with a thick layer 
 of scoriae, and afterwards by a stream of lava, were found 
 three years afterwards consolidated into ice, but not 
 melted. The city of Catania is constantly supplied 
 with ice from masses of snow which have been buried 
 under the ejections of Etna. 
 
 During the cooling down of lavas, the escape of 
 steam and various gases gives rise to the deposition of 
 many beautiful crystalline substances in the cavities 
 and on the surfaces of the lava. Deposits of sulphur, 
 specular-iron, tridymite, and many other substances 
 are often thus produced, and the colour and appearance 
 of the rock-masses are sometimes completely disguised 
 t by these surface incrustations, or by the decomposition 
 
SINKING OF SURFACES OF LAVA-STREAMS. Ill 
 
 of the materials of the lava by the action of the acid 
 gases., and vapours upon it. 
 
 Very frequently the surface of a lava-stream becomes 
 solid, while the deeper portions retain their fluid con- 
 dition ; under such circumstances the central portions 
 
 Fin. 32. TRANSVERSE SECTION OF A LAVA STREAM. 
 (The dotted line indicates the oiiginal surface.) 
 
 may flow away, leaving a great hollow chamber or cavern. 
 In consequence of this action, we not unfrequently find 
 the upper surface of a lava-current exhibiting a de- 
 pression, due to the falling in of the solidified upper 
 portions w r hen the liquid lava has flowed away and left 
 it unsupported, as in fig. 32. 
 
112 VOLCANOES. 
 
 CHAPTER V. 
 
 THE INTERNAL STRUCTURE OF VOLCANIC MOUNTAINS. 
 
 NEAR the high-road which passes between the towns 
 of Eger and Franzenbad in Bohemia, there rises a small 
 hill known as the Kammerbiihl (see fig. 33), which has 
 attracted to itself an amount of interest and attention 
 quite out of proportion to its magnitude or importance. 
 During the latter part of the last century and the 
 earlier years of the present one, the fiercest contro- 
 versies were waged between the partisans of rival 
 schools of cosmogony over this insignificant hill ; some 
 maintaining that it originated in the combustion of 
 a bed of coal, others that its materials were entirely 
 formed by some kind of ' aqueous precipitation,' and 
 others again that the hill was the relic of a small vol- 
 canic cone. 
 
 Among those who took a very active part in this 
 controversy was the poet Goethe, who stoutly main- 
 tained the volcanic origin of the Kammerbiihl, styling 
 it ' a pocket edition of a volcano.' To Groethe belongs 
 the merit of having suggested a very simple method 
 by which the controversies concerning this hill might 
 
THE KAMMEKBUHL. 
 
 113 
 
 be set at rest : he proposed that a series of excavations 
 should be undertaken around the hill, and a tunnel 
 driven right under its centre. The poet's friend, Count 
 Casper von Sternberg, determined to put this project 
 
 FIG. 33. THE KAMMERBUHL OK KAMMERBERG, BOHEMIA. 
 (As seen from the south-west.) 
 
 into execution. This series of excavations, which was 
 completed in 1837, has for ever set at rest all doubts 
 as to the volcanic origin of the Kammerbuhl. A plug 
 of basalt was found filling the centre of the mass, and 
 connected with a small lava-stream flowing down the 
 side of the hill ; while the bulk of the hill was shown 
 to be composed of volcanic scoriae and lapilli. The 
 
114 
 
 VOLCANOES. 
 
 section fig. 34 will illustrate the structure of the hill 
 as revealed by these interesting excavations. 
 
 It can of course very seldom happen that actual 
 mining operations, like those undertaken in the case of 
 the Kammerbiihl, will be resorted to in order to deter- 
 mine the structure of volcanic mountains. Geologists 
 have usually to avail themselves of less direct, but by 
 no means less certain, methods than that of making 
 
 FIG. 34. SECTIOX OF THE KAMMERBUHL, ix BOHEMIA. 
 
 a a. Metamorphic rocks, b. Basaltic scoriae, c. Solid plug of basalt rising 
 through the centre of the volcanic pile, d d. Lava-str< am composed of the 
 same rock, e e. Alluvial matter surrounding the old volcano. 
 
 (The dotted lines indicate the probable former outline of the volcano.) 
 
 artificial excavations in order to investigate the earth's 
 crust. Fortunately it happens that what we cannot 
 accomplish ourselves, nature does for us. The action 
 which w T e call ' denudation ' serves as a scalpel to dissect 
 volcanic mountains for us, and to expose their inner 
 recesses to our view. Many portions of the earth's 
 surface are complete museums crowded with volcanic 
 'subjects,' exhibiting every stage of the process of 
 dissection. In some, rains and winds have stripped off 
 the loose covering of cinders and dust, and exposed the 
 
VOLCANOES DISSECTED BY DENUDATION. 115 
 
 harder and more solid parts the skeleton of the moun- 
 tain. In others, the work of destruction has proceeded 
 still farther, and slowly wearing rivers or the waves of 
 the sea may have cut perfect, vertical sections of the 
 mountain-mass. Sometimes the removal of the mate- 
 rials of the volcanic mountain has gone on to such an 
 extent that its base and ground-plan are fully exposed. 
 It only requires the necessary skill in piecing together 
 our observations on these dissected volcanoes, in order 
 to arrive at just views concerning the 'comparative 
 anatomy ' of volcanoes. As the knowledge of the 
 structure of animals remained in the most rudimentary 
 condition until the practice of dissection was com- 
 menced, so our knowledge of volcanoes was likewise 
 exceedingly imperfect till geologists availed themselves 
 of the opportunities afforded to them of studying natu- 
 rally dissected volcanic mountains. 
 
 In some cases we may find that the sea has en- 
 croached v on the base of a volcanic hill, till one half of 
 it has been washed away, and the structure of the mass 
 to its very centre is exposed to our view. Thus in 
 fig. 6 (page 43), it will be seen that there lies in front 
 of Vulcano a peninsula called Vulcanello, consisting of 
 three volcanic cones, united at their base, with the lava- 
 streams which have flowed from them. One half of 
 the cone on the left-hand side of the picture has been 
 completely washed away by the sea, and a perfect sec- 
 tion of the internal structure of the cone is exposed. 
 The appearances presented in this section are shown in 
 
116 
 
 VOLCANOES. 
 
 the sketch, fig. 35. Some portions of the face of this 
 section are concealed by the heaps of fragments which 
 have fallen from it, but enough is visible to convince 
 us that three kinds of structures go to make up the 
 cone. In the first place, we have the loose scoriae and 
 lapilli, which in falling through the air have arranged 
 
 FIG. 35, 
 
 -NATURAL SECTION OF A VOLCANIC COSE IN THE ISLAND OF 
 VULCANO 
 
 . Crater, bb. Lava-streams, c. Dykes which have clearly formed the ducts, 
 through which the lava has risen to the crater, d d. Stratified volcanic scoriae. 
 e. Talus of fallen materials. 
 
 themselves in tolerably regular layers upon the sides 
 of the cone. In the second place, we have lava-streams 
 which have been ejected from the crater or from fissures 
 on the flanks of the cone, and flowed down its sides. 
 And thirdly, we find masses of lava filling up cracks in 
 the cone ; these latter are called ' dykes.' Of these 
 three kinds of structures most volcanic mountains are 
 
INTERNAL STRUCTURE OF VOLCANIC CONES. 117 
 
 built np, but in different cases the part played by these 
 several elements may be very unequal. Sometimes 
 volcanoes consist entirely of fragmentary materials, 
 at others they are made up of lavas only, while in the 
 majority of cases they have been formed by alterna- 
 tions of fragmentary and fluid ejections, the whole 
 being bound together by dykes, which are masses of 
 lava injected into the cracks formed from time to time 
 in the sides of the growing cone. 
 
 If we direct our attention in the first place to the 
 fragmentary ejections, we shall find that they affect a 
 very marked and peculiar arrangement, which is best 
 exhibited in those volcanic cones composed entirely of 
 such materials. 
 
 Everyone who examines volcanoes for the first time 
 will probably be struck by the regular stratification of 
 materials of which they are composed. Thus the tuffs 
 covering the city of Pompeii are found to consist of 
 numerous thin layers of lapilli and volcanic dust, 
 perfectly distinct from one another, and assuming even 
 the arrangement which we usually regard as charac- 
 teristic of materials that have been deposited from a 
 state of suspension in water. The fragmentary mate- 
 rials in falling through the air are sorted, the finer 
 particles being carried farther from the vent than the 
 larger and heavier ones. The force of different volcanic 
 outbursts also varies greatly, and sometimes materials 
 of different character are thiown out during successive 
 ejections. These facts will be illustrated by fig. 36, 
 
118 VOLCANOES. 
 
 which is a drawing of a section exposed in a quarry 
 opened in the side of the Kammerbiihl. In this section 
 we see that the falling scoriae have been arranged in 
 rudely parallel beds, but the regular deposition of these 
 has been interrupted by the ejection of masses of burnt 
 slate torn from the side of the vent, probably during 
 some more than usually violent paroxysm of the volcano. 
 In those volcanoes which are built up of tuffs and 
 
 ^Tgr^- ^>-g*y- C>\ 'c&- '*? 
 y^^t^-^JT^ 
 
 
 _!* 
 
 -<y ^f&a&jf&'-'y-tfszaf 
 
 FIG. 36. SECTION IN THE SIDE OF THE KAMMERBUHL, BOHEMIA. 
 
 a a. Stratified basaltic scoriae, b b. Bands made up of fragments of burnt slate. 
 c. Stratified basaltic scoriae, d d. Pseudo-dykes occupying lines of fault. 
 
 materials which have fallen in the condition of a muddy 
 paste, the perfect stratification of the mass is often very 
 striking indeed, and large cones are found built up of 
 thin uniformly-spread layers of more or less finely- 
 divided materials, disposed in parallel succession. Such 
 finely- stratified tuff-cones abound in the district of the 
 Campi Phlegrsei. 
 
 If, in consequence of any subterranean movements, 
 fissures are produced in the sides of the cones formed 
 
ARRANGEMENT OF FBAGMENTAL MATERIALS. 119 
 
 of fragmentary materials, these often become gradually 
 filled with loose fragments from the sides of the fissure, 
 and in this manner ' pseudo-dykes ' are formed. An 
 example of such pseudo-dykes is represented in fig. 36, 
 where the beds composing the volcanic cone of the 
 Kammerbuhl are seen to have been broken across or 
 faulted, and the fissures produced in the mass have 
 been gradually filled with loose fragments. 
 
 It is not difficult to imitate, on a small scale, the 
 conditions which exist at those volcanic vents from 
 which only fragmentary materials are ejected. If we 
 take a board having a hole in its centre, into which a 
 pipe is inserted conveying a strong air-blast, we shall, 
 by introducing some light material like bran or saw- 
 dust into this pipe cause an ejection of fragments, 
 which will, when the board is placed horizontally, 
 fall around the orifice of the pine and accumulate there 
 in a conical heap (fig. 37). It will be found necessary, 
 as was shown by Mr. Woodward, who performed the 
 experiment before the Physical Society, to adopt some 
 contrivance, such as a screw, for forcing the material 
 into the air-pipe. If we alternately introduce materials 
 of different colours, like mahogany- and deal-sawdust 
 into the pipe, these materials will be arranged in layers 
 which can be easily recognised, and the mode of accu- 
 mulation of the mass will be evident. By means of a 
 sheet of tin or cardboard we may divide this miniature 
 volcanic cone vertically into two portions, and if we 
 
120 
 
 VOLCANOES. 
 
 sweep one of these away the internal structure of the 
 other half will be clearly displayed before our eyes. 
 
 In this way we shall find that the conical heap of 
 sawdust with the hole in its centre has a very peculiar 
 and definite arrangement of its materials. It is made up 
 of a number of layers each of which slopes in opposite 
 directions, toivards the centre of ejection and away 
 
 FIG. 37. EXPERIMENTAL ILLUSTRATION OF THE MODE OK FORMATION 
 OF VOLCANIC CONES COMPOSED OF FRAGMENTAL MATERIALS. 
 
 from that centre. These layers are thickest along the 
 line of the circle where the change in slope takes place, 
 and they thin away in the direction of the two opposite 
 slopes. 
 
 The cause of this peculiar arrangement of the 
 materials is evident. The sawdust thrown up by the 
 air blast descends in a shower and tends to accumulate 
 in a circular heap around the orifice, the area of this 
 
CAUSE OF THIS ARRANGEMENT. 121 
 
 circular heap being determined by the force of the 
 blast. Within this circular area, however, the quantity 
 of falling fragments is not everywhere the same ; along 
 a circle surrounding the vent at a certain distance, the 
 maximum number of falling fragments will be found to 
 descend, and here the thickest deposit will take place. 
 As this goes on, a circular ridge will be formed, with 
 slopes towards and away from the centre of injection. 
 As the ridge increases in height, the materials will tend 
 
 FIG. 38. NATURAL SECTION OF A TUFF-CONE FORMING Tin: CAPE OF 
 
 MlSENUM, AND EXHIBITING THE PECULIAR INTERNAL ARRANGE- 
 MENT CHARACTERISTIC OF VOLCANOES COMPOSED OF FRAGMENTARY 
 MATERIALS. 
 
 to roll down either one slope or the other, and gradually 
 a structure of the form shown in the figure will be piled 
 up. The materials sliding down the outer slope will 
 tend to increase the area of the base of the cone, while 
 those which find their way down the inner slope will 
 fall into the vent to be again ejected, 
 
 Volcanic cones composed of scoriae, dust, &c. are 
 found to have exactly the same internal structure as is 
 exhibited by the miniature cone of sawdust. The more 
 
122 
 
 VOLCANOES. 
 
 or less regular layers of which they are made up dip in 
 opposite directions, away from and towards the vent, 
 and thin out in the direction of their dip (see fig. 38). 
 In small cones the crater or central cavity is of con- 
 siderable size in proportion to the whole mass, but as 
 the cone grows upwards and outwards, the dimensions 
 of the crater remain the same, while the area of the 
 base and the height of the cone are continually increas- 
 
 
 FIG. 39. SECTION OF A SMALL SCORIA-CONE FORMED WITHIN THE 
 CRATER OF VESUVIUS IN THE YEAR 1835, ILLUSTRATING THE 
 FILLING-UP OF THE CENTRAL VENT OF THE CONE BY SUBSEQUENT 
 EJECTIONS. 
 
 ing. This is the normal structure of volcanic cones 
 formed of fragmentary materials, though, as we shall 
 hereafter show, many irregularities are often produced 
 by local and temporary causes. 
 
 In some cases the central vent of a volcanic scoria- 
 cone may be filled up by subsequent ejections. A 
 beautiful example of this kind was observed by Abich, 
 
BREACHED CONES. 
 
 123 
 
 in the case of a small cone formed within the crater of 
 Vesuvius in 1835, and is represented in fig. 39. 
 
 Many cones formed in the first instance of scoriae, 
 tuff, and pumice may give rise to streams of lava, be- 
 fore the vent which they surround sinks into a state 
 of quiescence. In these cases, the liquid lava in the 
 vent gives off such quantities of steam that masses of 
 froth or scoriae are formed, which are ejected and 
 accumulate around the orifice. When the force of the 
 explosive action is exhausted, the lava rises bodily in 
 
 FIG. 40. VOLCANIC COXES COMPOSED OF SCORIAE, AND BREACHED ON 
 ONE SIDE BY THE OUTFLOW OF LAVA-CURRENTS. 
 
 the crater, which it more or less completely fills. But, 
 eventually, the weaker side of the crater-wall yields 
 beneath the pressure of the liquid mass, and this part 
 of the crater and cone is swept away before the ad- 
 vancing lava-stream. Examples of such ' breached 
 cones ' abound in Auvergne and many other volcanic 
 districts (see fig. 40). A beautiful example of a cone 
 formed of pumice, which has been breached by the 
 outflow of a lava-stream of obsidian, occurs in the 
 Lipari Islands, at the Eocche Eosse. It is this locality 
 
124 VOLCANOES. 
 
 which supplies the whole world with pumice (see 
 fig. 41). 
 
 It is often surprising to find how volcanic cones com- 
 posed of loose materials, such as tuffs, scoriae, or pumice, 
 retain their distinctive forms, and even the sharp- 
 ness of their outlines, during enormous periods of time. 
 Thus, in the scoria-cones which abound in the Auvergne, 
 and were, in all probability, formed before the historical 
 period, the sharp edges of the craters appear to have 
 suffered scarcely any erosion, and the cones are as per- 
 
 FIG. 42. VOLCANIC CONES IN AUVERGNE WHICH HAVE SUFFERED TO 
 SOME EXTENT FROM ATMOSPHERIC DENUDATION. 
 
 feet in their outlines as though formed but yesterday. 
 It is probable that the facility with which these cindery 
 heaps are penetrated by the rain which falls upon them 
 is the cause why they are not more frequently washed 
 away. 
 
 Sometimes, however, scoria-cones are found reduced 
 by atmospheric waste to mere heaps of cinders, in 
 which the position of the crater is indicated only by a 
 slight depression, as in fig. 42. 
 
 When but little explosive action takes place at the 
 volcanic vent, and only fluid lava is ejected, mountains 
 
CONES COMPOSED OF LAVA. 125 
 
 are formed differing very greatly in character from the 
 cones composed of fragmentary materials. 
 
 If the lavas be of very perfect liquidity, like those 
 erupted in the Sandwich Islands, they flow outwards 
 around the vent to enormous distances. By the accu- 
 mulation of materials during successive outbursts, a 
 conical mass is built up which has but a slight eleva- 
 tion in proportion to the area of its base. Thus in Hawaii 
 we find great volcanic cones, composed of very fluid 
 lavas, which have a height of nearly 14,000 feet with a 
 diameter of base of seventy miles. In these Hawaiian 
 mountains the slope of the sides rarely exceeds 6 to 8. 
 
 But if, on the other hand, the lavas be of much 
 more viscid consistency, the character of the volcanic 
 cones which are produced by their extrusion will be 
 very different. The outwelling material will tend to 
 accumulate and heap itself up around the vent. By 
 successive ejections the first-formed shell is forced up- 
 wards and outwards, and a steep-sided protuberant mass 
 is formed, exhibiting in its interior a marked concentric 
 arrangement. Dr, Ed. Keyer, of Gratz, has devised a 
 very ingenious method for reproducing on a miniature 
 scale the characteristic features of these eruptions of 
 viscid lavas. He takes a quantity of plaster of Paris 
 reduced to a pasty consistence, which he forces through 
 a hole in a board. The plaster accumulates in a great 
 rounded boss about the orifice through which it has 
 been forced. If the plaster have some colouring matter 
 introduced into it, the mass, on being cut across, will 
 
126 
 
 VOLCANOES. 
 
 exhibit in the disposition of its colour-bands the kind 
 of action which has gone on during its extrusion, fig. 43. 
 There are many volcanic cones which exhibit clear 
 
 FIG. 43. EXPERIMENTAL ILLUSTRATION OF THE MODE OF FORMATION 
 OF VOLCANIC CONES COMPOSED OF VISCID LAVAS. 
 
 evidence of having thus been formed by the extrusion 
 of a viscid mass of lava through a volcanic fissure. 
 Among such we may mention the domitic Puys of 
 
 FIG. 44. THE GRAND PUT OF SARCOUI, COMPOSED OF TRACHYTE, RISING 
 BETWEEN TWO BREACHED SCORIA-CONES (AUVERGNE). 
 
 Auvergne, fig. 44, many andesitic volcanoes in Hun- 
 gary, the phonolite hills of Bohemia, and the so-called 
 ' mamelons ' of the Island of Bourbon. See figs. 45 and 
 46. When the interior of these masses is exposed by 
 
INTERNAL STRUCTURE OF LAVA-CONES. 
 
 12V 
 
 natural or artificial sections, they are all found to ex- 
 hibit the onion-like structure which occurs in the 
 plaster models. 
 
 Fio. 45. VOLCANIC CONE (MAMELON) COMPOSED OF VERY VISCID LAVA. 
 (ISLAND OF BOURBON.) 
 
 But while some volcanoes are composed entirely of 
 the fragmentary ejections and others are wholly formed 
 
 FIG. 46. ANOTHER MAMELON IN THE ISLAND OF BOURBON, WITH A 
 CRATER AT ITS SUMMIT. 
 
 by successive outflows of lava, the majority of vol- 
 canoes, especially those of larger dimensions, are built 
 up of alternations of these different kinds of materials. 
 
128 
 
 VOLCANOES. 
 
 The structure of these composite cones may be 
 understood by an inspection of the accompanying fig. 
 47, which shows the appearances presented in a cliff 
 on the coast of the Island of Madeira. We see that the 
 mass is made up of numerous layers of volcanic scoriae, 
 alternating with sheets of lava. The latter, which are 
 
 FIG. 47. CLIFF-SECTION TN THE ISLAND OF MADEIRA, SHOWING HOW 
 A COMPOSITE VOLCANO IS BUILT UP OK LAVA-STKEAMS, BEDS OF 
 SCORI/E, AND DYKES. 
 
 represented in transverse section in the drawing, are 
 seen to thin out on either side, and to vary greatly in 
 breadth. Besides the alternating masses of scoriae and 
 the lava-sheets, there are seen in the section, bands of 
 a bright-red colour, which are represented in the draw- 
 ing by black lines. These are layers of soil, or vol- 
 canic dust, which, by the passage of a lava-stream over 
 
NATURAL SECTIONS OF CONES. 129 
 
 their surface, have been burnt so as to acquire a brick- 
 red colour. These bands of red material, to which the 
 name of 'laterite' has been frequently applied, very 
 commonly occur in sections of composite volcanic cones. 
 Crossing the whole of the horizontally-disposed masses 
 in the section, we find a number of 4 dykes,' which are 
 evidently great cracks filled with lava from below. 
 Some of these run vertically through the cliffs, others 
 obliquely. In some cases the lava, rising to fill a dyke, 
 has flowed as a lava-stream at the surface. Last of all, 
 we must call attention to the fact that the section ex- 
 hibits evidence of great movements having taken place 
 subsequently to the accumulation of the whole of the 
 materials. A great crack has been produced, on one 
 side of which the whole mass has subsided bodily, 
 giving rise to the phenomenon which geologists call 
 a < fault.' 
 
 In the section, fig. 27, p. 104, copied from a drawing 
 of a sea-cliff in the Island of Vulcano, a transverse 
 section of a lava-stream is represented on a somewhat 
 larger scale. The upper and under surface of the lava- 
 stream is seen to have a scoriaceous structure, but the 
 thick central mass is compact, and divided by regular 
 joint-planes. This section also illustrates the fact that, 
 before the lava-stream flowed down the sides of the 
 mountain, a valley had been cut by meteoric agencies 
 on the flanks of the volcano, the dykes which traverse 
 the lower beds of tuff being abruptly truncated. 
 
 In mountain ravines, upon the slopes of ancient 
 7 
 
130 
 
 VOLCANOES. 
 
 volcanoes, and in the cliffs of volcanic islands, we are 
 often able to study the way in which these great moun- 
 
 Trachyte lava 
 Pumiceous tuff 
 Columnar phonolite 
 
 Basaltic scorise 
 
 Basalt 
 
 Trachyte 
 tuff 
 
 FIG. 48. SECTION SEEN AT THE CASCADE, BAINS DU MONT DORE. 
 
 tain masses are built up of alternating lava-currents, 
 beds of volcanic agglomerate, scoriae, tuff and dust, and 
 
 FIG. 49. SECTION IN THE ISLAND OF VENTOTIENNE, SHOWING A GREAT 
 STREAM OF ANDESITIC LAVA OVERLYING STRATIFIED TUFFS. 
 
 intersecting dykes. In fig. 48, the features above 
 described are illustrated by a section in the sides of 
 the great volcano of Mont Dore. 
 
SECTIONS IN THE PONZA ISLANDS. 
 
 131 
 
 In figs. 49, 50, 51, and 52, we have given 
 drawings of portions of the sea-cliffs in several of 
 
 FIG. 50. CLIFF ON THE SOUTH SIDE OF THE ISLAND OF SAN 
 STEPHANO. 
 
 a. Trachyte lava-stream, with a scoriaceous upper surface overlaid by stratified 
 tuffs, 6. 
 
 FIG. 51. THE HEADLAND OF MONTE DELLA GUARDIA, IN THE ISLAND 
 OF PONZA. 
 
 a. Columnar trachyte, b. Stratified tuffs, c. Pumiceous agglomerates. 
 d. Dyke of rhyolite. 
 
 the Ponza Islands, a small volcanic group off the 
 Italian coast. 
 
132 
 
 VOLCANOES. 
 
 c a, c a IT 
 
 FIG. 52. WESTERN SIDE OF THE SAME HEADLAND, AS SEEN FROM THE 
 NORTH SIDE OF LUNA BAY. 
 
 a. Trachyte lava. 6. Stratified tuffs, c. Dykes of rhyolite, with their edges passing 
 into pitchstone. d. Pumiceous agglomerate. 
 
 Fig. 53 represents a cliff-section in the island of 
 Salina, one of the Liparis, exhibiting evidence that a 
 
 FIG. 53. SEA-CLIFF AT IL CAPO, THE NORTH-EAST POINT OF SALINA, 
 SHOWING STRATIFIED AGGLOMERATES TRAVERSED BY NUMEROUS 
 DYKES, THE WHOLE BEiNG UNCONFORMABLY OVERLAID BY STRATI- 
 FIED AQUEOUS DEPOSITS. 
 
PAKT PLAYED BY DYKES IN CONE-BUILDING. 133 
 
 series of volcanic agglomerates traversed by dykes of 
 Andesite have been denuded and covered by a recent 
 stratified deposit. 
 
 In the formation of these great composite cones, a 
 minor but by no means insignificant part is played by 
 the dykes, or lava-filled fissures, which are seen 
 traversing the mass in all directions. That dyke- 
 fissures often reach the surface of a volcanic cone, and 
 that the material which injects them then issues as a 
 lava-stream, is illustrated by fig. 54. The formation 
 
 FIG. 54. SECTION OBSERVED IN THE VAL DEL BOVE, ETNA, SHOWING 
 A BASALTIC DYKE, FROM THE UPPER PAR!' OF WHICH A LAVA- 
 CURRENT HAS FLOWED. 
 
 of these cracks in a volcanic cone, and their injection 
 by liquid lava, must of course distend the mountainous 
 mass and increase its volume. If we visit the great 
 crater-walls of Somma in Vesuvius, and of the Val del 
 Bove in Etna, we shall find that the dykes are so 
 numerous that they make up a considerable portion of 
 the mass. When the loose scoriae and tuffs are re- 
 moved by denudation, these hard dykes often stand 
 up prominently like great walls, as represented in 
 fig. 55. Even in such cases as these, however, it is 
 doubtful whether the bulk of all the dykes put together 
 
134 
 
 VOLCANOES. 
 
 exceeds one-tenth of that of the lavas and fragmen- 
 tary materials. 
 
 Hence we are led by an examination of the internal 
 structure of volcanic mountains to conclude that scoria?- 
 and tuff-cones, and cones formed of very liquid lavas, 
 increase by an exogenous mode of growth, all new 
 materials being added to them from without; in the 
 
 FIG. 55. BASALTIC DYKES PROJECTING FROM MASSES OF STRATIFIED 
 SCORIAE IN THE SIDES OF THE VAL DEL BOVE, ETNA. 
 
 cones formed of very viscid lavas, on the other hand, 
 the growth is endogenous, taking place by successive 
 accretions within it. The composite cones owe their 
 origin to both the exogenous and the endogenous 
 modes of growth, but in a much greater degree to the 
 former than the latter. The layers of scoriae, tuff, and 
 dust, and the successive lava-streams are added to the 
 
THEORY OF ELEVATION CRATEES. 135 
 
 mass from without, and the lava forming the dykes 
 from within it. 
 
 . There are doubtless cases in which, when a tuff-cone 
 is formed, a mass of very viscid lavas is extruded into 
 its interior, and the mass is distended like a gigantic 
 bubble. But inasmuch as the very viscid lavas do not 
 appear to give rise to scoriae to anything like the same 
 extent as the more liquid kinds, such ' cupolas,' as 
 they have been called by some German geologists, are 
 probably not very numerous, and may be regarded as 
 constituting the exception rather than the rule. The 
 idea which was formerly entertained by some geologists 
 that all great volcanic mountains were formed of masses 
 originally deposited in a horizontal position, and subse- 
 quently blown up into a conical form, has been effectu- 
 ally disposed of by the observations of Lyell and Scrope. 
 The condition of the great fluid masses which 
 underlie volcanic vents is another point on which 
 much light has been thrown by the study of naturally- 
 dissected volcanoes. In some cases, as was shown by 
 Hochstetter during his admirable researches among 
 the New Zealand volcanoes, the rising lavas form a 
 great chamber for themselves in the midst of a vol- 
 canic cinder-cone, taking the place of loose materials 
 which are re-ejected from the vent, or have been 
 re-fused and absorbed into the mass of lava itself. 
 From this central reservoir of lava, eruptions are kept 
 up for some time, but when the volcano sinks into a 
 state of quiescence the lava slowly consolidates. In 
 
136 VOLCANOES. 
 
 such slowly solidified masses of lava, very beautiful 
 groups of radiating columns are often exhibited. 
 Northern Germany abounds with examples of such 
 basaltic masses, which have once formed the centres of 
 great cinder-cones ; but in consequence of the removal 
 of the loose materials and the surrounding strata by 
 denudation, these central reservoirs of the volcanoes 
 have been left standing above the surface, and exhibit 
 the peculiar arrangements of the columns formed in 
 them during the process of cooling. 
 
 But in the majority of the more solidly-built com- 
 posite volcanoes no such liquid reservoir can be formed 
 within the volcanic cone itself. Under these circum- 
 stances, the lavas, especially those of more liquid 
 character, tend to force passages for themselves among 
 the rocks through which they are extruded, Wherever 
 a weak point exists, there such lavas will find their 
 way, and as the planes of stratification in sedimentary 
 rocks constitute such weak places, we constantly find 
 sheets of lava thus inserted between beds of aqueous 
 origin. The areas over which these intrusive sheets 
 of rock sometimes extend may be very great, but the 
 more fusible, basic lavas (basalt, &c.) usually form 
 much more widely-spreading sheets than the less 
 fusible, acid lavas. In some cases these great intrusive 
 sheets are found extending to a distance of twenty or 
 thirty miles from the centre at which they were 
 ejected, and they often follow the bedding of the strata 
 with which they are intercalated in so regular a manner, 
 
INTRUSIVE LAVA-SHEETS. 
 
 137 
 
 that it is difficult for an observer to believe at first 
 sight that they can have been formed in the way 
 which we have described. A closer examination will 
 generally reveal the fact that while these intrusive 
 lava-sheets retain their parallelism with the strata 
 among which they have been intruded, over consider- 
 able areas, yet they sometimes break across, or send 
 offshoots into them, as shown in fig. 56. In all cases, 
 
 FIG. 56. SHEETS OF IGNEOUS ROCK (BASALT) INTRUDED BETWEK.V 
 BEDS OF SANDSTONE, CLAY, AND LIMESTONE. (ISLAND OF SlCYE.) 
 
 too, the rocks lying above and below such sheets will 
 be found to be more or less baked and altered, and 
 this affords a very convincing evidence of the in- 
 trusion of the igneous mass between the strata so 
 altered. 
 
 That in the case of most great volcanic mountains, 
 or systems of mountains, vast reservoirs of liquid lava 
 must exist in the earth's crust far below the surface, 
 there can be little room for doubt. Whether such 
 fluid masses are in direct or indirect communication 
 with a great central reservoir, even supposing such to 
 
138 VOLCANOES. 
 
 exist, is a totally different question. In many cases 
 the outburst of volcanoes in more or less close proximity 
 has been observed to take place simultaneously, while 
 in others the commencement of the eruption of one 
 volcano has coincided with the lapse into quiescence 
 of another in its vicinity. On the other hand, the re- 
 markable case of the volcanoes of Hawaii seems to in- 
 dicate that two vents in close proximity may be supplied 
 from perfectly distinct reservoirs of lava. The active 
 craters of Mauna Loa and Kilauea are situated at the 
 heights of 14,000 and 4,000 feet respectively above the 
 sea level; yet the former is sometimes in a state of 
 violent activity, with which the latter shows no signs 
 of sympathy whatever. We shall, in* a future chapter, 
 adduce evidence that the liquid lavas in underground 
 reservoirs may undergo various stages of change in the 
 enormous periods of time during which habitual vol- 
 canic vents are supplied from them. 
 
 We have already shown that the character assumed 
 by a mass of fused material in cooling varies greatly 
 according as the cooling takes place rapidly at the 
 surface or slowly under enormous pressure. In the 
 former case a glassy base is formed containing a greater 
 or smaller number of crystallites or embryo crystals, in 
 the latter the whole rock is converted into a mass of 
 fully-developed crystals. 
 
 The lavas which are poured out at the surface con- 
 sist, as we have seen, of a glassy magma in which a 
 greater or smaller number of crystals are found which 
 
CONSOLIDATION OF LAVAS AT GREAT DEPTHS. 139 
 
 have been floated up from below. The great dykes and 
 intrusive sheets consist for the most part of a mass of 
 small or imperfectly developed crystals in which a 
 number of large and perfectly formed crystals are em- 
 bedded. Such rocks are said to have a < porphyritic ' 
 structure. The rocks formed by the consolidation of 
 the liquid .masses in the underground reservoirs are 
 found to be perfectly crystallised, the crystals impressing 
 one another on every side and making up the whole 
 mass to the exclusion of any paste or magma between 
 them. The crystals in those rocks which have con- 
 solidated at these vast depths exhibit evidence, in their 
 enclosed watery solutions and liquefied carbonic acid, 
 of the enormous pressures under which they must have 
 been consolidated. The lavas, the more or less porphy- 
 ritic rocks of the dykes and sheets, and the perfectly 
 crystalline (granitic) rocks of the underground reservoirs 
 pass into one another, however, by the most insensible 
 gradations. 
 
 We sometimes find examples of volcanoes which, by 
 the action of denuding forces, have had their very 
 foundations exposed to our view. Such examples occur 
 in the Western Isles of Scotland, in the Euganean Hills 
 near Padua in Northern Italy, and in many other parts 
 of the earth's surface. In these cases we are able to 
 trace the ground-plan of the volcanic pile, and to study 
 the materials which have consolidated deep beneath 
 the surface in the very heart of the mountain. 
 
 In studying these ' basal wrecks ' of old volcanoes 
 
140 VOLCANOES. 
 
 it is always necessary to bear in mind that the appear- 
 ance and general characters of a volcanic rock may be 
 completely disguised by chemical changes going on 
 within it. It is through want of attention to this fact 
 that so many mistakes were made by the Wernerian 
 school of geologists who declared that they could find 
 no analogy between the basaltic rocks of the globe and 
 the products of active volcanoes, and were hence led to 
 refer the origin of the former to some kind of < aqueous 
 precipitation.' 
 
 Many of the hard and crystalline marbles which are 
 employed as ornamental stones were originally loose 
 masses of shells and corals, as we easily perceive when 
 we examine the polished faces. But these incoherent 
 heaps of organic debris have been converted into a com- 
 pact and solid rock in consequence of the mass being 
 penetrated by water containing carbonate of lime in solu- 
 tion. Crystals of this substance were deposited in every 
 cavity and interstice of the mass, and thus the accu- 
 mulation of separate organisms was gradually trans- 
 formed to a material of great solidity and hardness. 
 
 In precisely the same way loose heaps of scoriae, 
 lapilli, or pumice may, by the passage through them of 
 water containing various substances in solution, have 
 their vesicles filled with crystals, and thus be converted 
 into the hardest and most solid of rock-masses. Simi- 
 larly the scoriaceous portions of lava-streams have their 
 vesicles filled with crystalline substances deposited from 
 a state of solution, and are thus converted into a solid 
 
FORMATION OF AMYGDALOIDS. 141 
 
 mass which may at first sight appear to offer but little 
 resemblance to the vesicular materials of recent lava- 
 streams. To these vesicular rocks which have their 
 cavities filled with crystalline substances geologists 
 apply the name of amygdaloids (L. amygdalus, an 
 almond). The cavities in lava-rocks are usually more 
 or less elongated, owing to the movement of the mass 
 while in a still plastic state, and the crystalline mate- 
 rials filling these cavities take the almond-like shape ; 
 hence the name. 
 
 When the amygdaloids and altered fragmentary 
 ejections of volcanoes are studied microscopically, their 
 true character is at once made manifest. The ex- 
 posure of faces of these altered volcanic rocks to the 
 weathering influences of the atmosphere, in many 
 cases also causes their true nature to be revealed, 
 the crystalline materials filling the interstices and 
 vesicles of the mass are dissolved away by the rain- 
 water containing carbonic acid, and the rock regains its 
 original cavernous structure and appearance. But this 
 repeated passage of water through volcanic rock-masses 
 may result in the removal of so large a portion of their 
 materials that the remainder crumbles down into the 
 condition of a clay or mud. 
 
 In the basal wrecks of volcanoes, of which we have 
 spoken, we usually find only small and fragmentary 
 remains of the great accumulations of loose and scoria- 
 ceous materials which originally constituted the bulk 
 of the mountain mass. In the centre of the ground- 
 
142 VOLCANOES. 
 
 plan of such a denuded volcano we find great masses 
 of highly crystalline or granitic rock, which evidently 
 occupy vast fissures broken through the sedimentary 
 or other rocks upon which the volcanic pile has been 
 reared. These highly crystalline rocks exhibit, as we 
 haTe shown, clear evidence of having been consolidated 
 from a state of fusion with extreme slowness and under 
 enormous pressure, but their ultimate chemical com- 
 position is identical with that of the lavas which have 
 been ejected from the volcano. 
 
 When, as frequently happens, the volcano, after 
 pouring out one kind of lava for a certain period, has 
 changed the nature of its ejections, and given rise to 
 materials of different composition, we find clear evi- 
 dence of the fact in studying the basal wreck or 
 ground-plan of the volcano. A great intrusive crys- 
 talline mass, of the same checiical composition as the 
 first-extruded lava, is found to be rent asunder and 
 penetrated by a similarly crystalline mass having the 
 composition of the lavas of the second period. Thus, 
 in the volcanoes of the Western Isles of Scotland, which 
 are reduced by the action of denudation to this con- 
 dition of basal wrecks, we find that rhyolites, trachytes, 
 and andesites were ejected during the earlier periods 
 of their history, and basalts during the later periods. 
 We find on studying the ground-plan of these vol- 
 canoes that great masses of granite, syenite, and diorite 
 the crystalline representatives of the first-extruded 
 lavas are penetrated by intrusions of gabbro the 
 
FIG. 57. PLAN OF THE DISSECTED VOLCANO OP MULL, 
 IN THE INNER HEBRIDES. 
 
 a Rocks, on 
 which the 
 Volcano 
 has been 
 built up. 
 
 b Great intru- 
 sive masses 
 of acid and 
 intermediate 
 
 rocks. 
 
 c Lava cur- 
 rents of ba- 
 salt which 
 have flowed 
 from d. 
 
 d Intrusive 
 masses of 
 gabbros & 
 dolerite. 
 
 e Lava cur- 
 rents which 
 have flowed 
 from b. 
 
 Volcanic 
 
 tuffs and 
 agglome- 
 rates. 
 
 FIG. 58. SECTION OF THE VOLCANO ALONG THE LINE A B. 
 
ANCIENT VOLCANO OF MULL. 143 
 
 granitic form of the later-ejected lavas. These features 
 are admirably illustrated by the ruined volcano now 
 constituting the Island of Mull, one of the Inner 
 Hebrides, a plan of which is given in fig. 57, and a sec- 
 tion in fig. 58. This volcano probably had a diameter 
 at its base of nearly thirty miles, and a height of from 
 10,000 to 12,000 feet, but is now reduced to a group of 
 hills few of which exceed 3,000 feet in height. 
 
 From these great intrusive masses of highly crystal- 
 line rocks there proceed in every direction great spurs 
 or dykes, which are evidently the radiating fissures 
 formed during the outwelling of igneous materials 
 from below, injected by these fluid substances. The 
 rock forming these dykes is often less perfectly crystal- 
 line than that which constitutes the centre of the mass, 
 and we may indeed find among the materials of these 
 dykes every variety of structure, from the perfectly 
 crystalline granite to the more or less glassy substance 
 of lavas. Besides the vertical or oblique 1 dykes we 
 also find horizontal sheets, which, passing from these 
 central masses, have penetrated between the sur- 
 rounding strata, often, as we have seen, to enormous 
 distances 
 
 For the sake of simplicity, we have spoken of these 
 ground-plans, or basal wrecks of volcanoes, as consti- 
 tuting a flat plain ; as a matter of fact, however, the 
 unequal hardness of the materials composing volcanic 
 mountains causes them to assume, under the influence 
 of denuding agencies, a very rugged and uneven sur- 
 
144 VOLCANOES. 
 
 face. The hard crystalline materials filling the central 
 vent stand up as great mountain groups; each large 
 dyke, by the removal of the surrounding softer mate- 
 rials, is left as a huge wall-like mass, while the rem- 
 nants of lava-streams are seen constituting a number 
 of isolated plateaux. 
 
 The great Island of Skye is the basal wreck of 
 another volcano which was also in eruption during 
 Tertiary times ; probably, many millions of years ago. 
 This immense volcano had originally a diameter at its 
 base of about thirty miles, and a height of 12,000 to 
 15,000 feet, and must have been comparable to Etna 
 or Teneriffe in its dimensions. At the present time, 
 there is nothing left of this vast pile but the highly 
 crystalline granites and gabbros filling up the great 
 fissures through which the eruption of igneous mate- 
 rials took place. These, worn by denudation into 
 rounded dome-like masses and wild rugged peaks, con- 
 stitute the T^ed Mountains and Coolin Hills of Skye, 
 which rise to the height of more than 3,000 feet above 
 the sea-level. From these great, central masses of 
 crystalline rocks, innumerable radiating dykes may be 
 found rising through the surrounding rock-masses, 
 with isolated patches of the scoriae and lapilli ejected 
 from the volcano, which have here and there escaped 
 removal by denudation. Along what were the outskirts 
 of this great mountain-mass are found flat-topped hills, 
 built up of lava-streams, only small portions of which 
 have escaped removal by denudation. 
 
BESERVOIES BENEATH VOLCANOES. 145 
 
 But this wearing away of the structure of a volcanic 
 cone by the denuding forces may proceed even one 
 stage farther, and we may then have revealed for our 
 inspection and study the mass of originally fluid 
 materials, from which one or more volcanoes have been 
 fed, cooled and consolidated in their original reservoir. 
 There are many examples of masses of granitic or 
 highly crystalline rocks, having precisely the same 
 composition as the different varieties of lavas, which 
 are found lying in the midst of the sedimentary rocks, 
 and sending off into these rocks veins and dykes of the 
 same composition with themselves. No one who has 
 carefully studied the appearances presented by volcanic 
 mountains in different stages of dissection, by the 
 action of denuding forces, can avoid recognising these 
 great granitic masses as the cooled reservoirs from 
 which volcanoes have in all probability been supplied 
 during earlier periods of the earth's history. 
 
 The eruption of these great masses of incandescent 
 rock, impregnated with water and acid gases, through 
 strata of limestone, sandstone, clay, coal, &c., may be 
 expected to produce striking and wonderful chemical 
 changes in the latter. Nor are we disappointed in 
 these anticipations. Whenever we examine the sedi- 
 mentary materials around volcanic vents, we find that, 
 in contact with the once-fused materials, they everywhere 
 exhibit remarkable evidences of the chemical action to 
 which they have been subjected. The limestones are 
 converted into statuary marble, the sandstones pass 
 
146 VOLCANOES. 
 
 into quartzite, the clays assume the hardness and lustre 
 of porcelain, while the coals have lost their volatile in- 
 gredients and assumed a form like coke or graphite. 
 And these changes are found to extend in many cases 
 to the distance of many hundreds of yards from the 
 planes of junction between the igneous and the sedi- 
 mentary materials. 
 
 Among the most interesting effects resulting from 
 the extrusion of masses of incandescent rock, charged 
 with water and various gases, through beds of lime- 
 stone, clay, sandstone, &c., we may mention the pro- 
 duction of those beautiful crystalline minerals which 
 adorn our museums and are so highly prized as gems. 
 By far the larger part of these beautiful minerals have 
 been formed, directly or indirectly, by volcanic agencies. 
 
 These gems and beautiful minerals are, for the most 
 part, substances of every-day occurrence, which entirely 
 owe their beauty to the crystalline forms they have 
 assumed. The diamond is crystallised carbon, the ruby 
 and sapphire are crystallised alumina, the amethyst and 
 a host of other gems are crystallised silica ; and in al- 
 most all cases the materials of gems are common and 
 widely diffused, it is only in their finely crystalline 
 condition that they are rare and therefore valuable. 
 
 Crystals are formed during the slow deposition of a 
 substance, either by the evaporation of a liquid in' which 
 it is dissolved, by its volatilisation, or its cooling from 
 a state of fusion. In many cases it can be shown that 
 the formation of large and regular crystals is aided if 
 
FOKMATION OF VOLCANIC MINERALS. 147 
 
 the work goes on with extreme slowness and under 
 great pressure. By sealing up various substances in 
 tubes containing water which can be k^pt at a high 
 temperature, minute crystals of many well-known mine- 
 rals have been artificially formed by chemists. Part of 
 the water converted into steam has formed a powerful 
 spring, which, re-acting upon the remainder of the 
 liquid in the tube, has subjected it to enormous pressure, 
 and under these conditions of extreme pressure and 
 temperature, chemical actions take place of which we 
 have no experience under ordinary circumstances. 
 The experiments of Mr. Hannay seem to prove that 
 when carbon is separated from certain organic sub- 
 stances at a high temperature and under great pressure, 
 it may crystallise in the form of the diamond. And 
 the recent discovery of diamonds in the midst of 
 materials filling old volcanic vents in South Africa 
 seems to show that this was in many cases the mode 
 in which the gem was originated. Even under the 
 conditions which prevail at the earth's surface, however, 
 minute and unnoticed chemical actions taking place 
 during long periods of time, produce most remarkable 
 results. This has been well illustrated by M. Daubree, 
 who has shown that in the midst of masses of concrete 
 which the Eomans built up around the hot springs of 
 Plombieres and other localities, many crystalline 
 minerals have been formed, in the course of 2,000 
 years, by the action of the waters upon the ingredients 
 of the concrete. 
 
148 VOLCANOES. 
 
 But most of the crystals of minerals which have 
 been thus artificially formed are of minute, indeed often 
 of microscopic, dimensions. In the underground re- 
 servoirs beneath volcanoes, however, we have all the 
 necessary conditions for the formation of crystals of 
 minerals on a far grander scale. High temperatures, 
 pressures far greater than any we can command at 
 the earth's surfkce, the action of superheated steam 
 and many acid gases on the various constituents of both 
 igneous and sedimentary rocks, and, above all, time of 
 almost unlimited duration ; these constitute such a set 
 of conditions as may fairly be expected to result in the 
 formation of crystals, similar to those artificially pro- 
 duced but of far greater size and beauty. 
 
 If we visit those parts of the earth's surface where 
 great masses of fused volcanic rock have slowly cooled 
 down in contact with sedimentary materials, we shall 
 not be disappointed in our expectations. Diamonds, 
 rubies, sapphires, emeralds, topazes, garnets, and a 
 host of equally beautiful, if less highly prized, crystalline 
 substances, are found in such situations, lying in the 
 subterranean chemical laboratories in which they have 
 been formed, but now, by the action of denuding forces, 
 revealed to our view. 
 
 In some cases it is not necessary to penetrate to 
 these subterranean laboratories in order to find these 
 beautiful gems and other crystallised minerals ; for the 
 steam jets which issue from volcanic fissures carry 
 up fragments of rock torn from the side of the vent, 
 
FORMATION OF MINEEAL-VEINS. 149 
 
 and in the cavities and fissures of such ejected masses 
 beautiful crystallised products are often found. Such 
 rock-fragments containing minerals finely crystallised are 
 found abundantly on the flanks of Vesuvius and other 
 active volcanoes, and among the materials of the 
 Laacher See and other extinct volcanoes. 
 
 But it is not only the finely crystallised minerals 
 and gems which we owe to volcanic action. The various 
 metallic minerals have nearly all been brought from 
 deep-seated portions of the earth's crust and deposited 
 upon the sides of rock-fissures by the agency of the 
 same volcanic forces. /It is these forces which have, in 
 the first instance, opened the cracks through the solid 
 rock masses ; and, in the second place, have brought 
 the metallic sulphides, oxides, and salts either in 
 fusion, in solution, or in a vaporised condition from 
 the deep-seated masses within the earth, causing them 
 to crystallise upon the sides of the fissures, and thus 
 form those metallic lodes and veins which are within 
 reach of our mining operations. 
 
 There is still one other important class of minerals 
 which owe their existence, though indirectly, to vol- 
 canic agencies. The cavities of igneous rocks, espe- 
 cially the vesicles formed by the escape of steam, con- 
 stitute, when filled with water, laboratories in which 
 complicated chemical reactions take place. The mate- 
 rials of the lava are gradually dissolved and re-crystal- 
 lised in new combinations. By this means the most 
 beautiful examples of such minerals as the agates, the 
 
150 VOLCANOES 
 
 onyxes, the rock-crystals, the Iceland- sjars, and the 
 numerous beautiful crystals classed together as 'Zeo- 
 lites ' have been formed. No one can visit a large 
 collection of crystalline minerals without being struck 
 with the large number of beautiful substances which 
 have thus been formed as secondary products from 
 volcanic materials. 
 
CHAPTER VI. 
 
 THE VARIOUS STRUCTURES BUILT UP AROUND VOLCANIC 
 VENTS. 
 
 FROM what has been said in the preceding chapters it 
 will be seen that while some of the materials ejected 
 from volcanic vents are, 'by the movements of the air 
 and ocean, distributed over every part of the face of 
 the globe, another, and by far the larger, part of the 
 matter so ejected, accumulates in the immediate vicinity 
 of the vent itself. By this accumulation of erupted 
 materials, various structures are built up around the 
 orifices from which the ejections take place, and the 
 size and character of these structures vary greatly in 
 different cases, according to the quantity and nature of 
 the ejected materials, and the intensity of the eruptive 
 forces by which they were thrown from the orifice. We 
 shall proceed in the present chapter to notice the chief 
 varieties in the forms and characters of the heaps of 
 materials accumulated round volcanic vents. 
 
 These heaps of materials vary in size from masses 
 no bigger than a mole-heap up to mountains like Etna, 
 Teneriffe, and Chimborazo. The size of volcanic moun- 
 
152 VOLCANOES. 
 
 tains is principally determined by the conditions of the 
 eruptive action at the vent around which they are 
 formed. If this action exhausts itself in a single effort, 
 very considerable volcanic cones, like the Monte Nuovo 
 with many similar hills in its vicinity, and the Puys of 
 Auvergne, may be formed ; but if repeated eruptions 
 take place at longer or shorter intervals from the same 
 vent, there appears to be scarcely any limit to the size 
 of the structures which may, under such conditions, be 
 formed. It is by this repeated action from the same 
 volcanic vent going on for thousands or even millions 
 of years, that the grandest volcanic mountains of the 
 globe have been built up. Such volcanoes have some- 
 times a diameter at their base of from 30 to 100 miles, 
 and an elevation of from 10,000 to 25,000 feet. 
 
 The form of volcanic mountains is determined in 
 part by the nature of the materials ejected, and in part 
 by the character of the eruptive action. 
 
 From what has been said in the preceding chapter, 
 it will be gathered that the volcanoes built up by ejec- 
 tions of fragmentary materials differ in many striking 
 particulars from those formed by the out welling of lavas 
 from volcanic vents. In a less degree, the volcanoes 
 composed of the same kind of volcanic materials also 
 vary among themselves. 
 
 When masses of scoriae in a semi-fluid condition are 
 thrown to only a little distance above the volcanic vent, 
 so that they have not time to assume a perfectly solid 
 condition before they fall round the vent, the rugged 
 
CHARACTERS OF SCORIA-CONES. 153 
 
 masses of lava unite to form heaps of most irregular 
 shape. lu such cases, the falling fragments being in a 
 semi-plastic state, stick to the masses below, and do not 
 tend to roll down the sides of the heap. Irregular heaps 
 of such volcanic scoriae abound on the surfaces of lava- 
 streams, being piled up around each 4 bocca ' or vent 
 which the steam-jets escaping from the lava-currents 
 form at their surfaces. Such irregular accumulations 
 of scoriae were observed on the lavas of Vesuvius during 
 the eruptions of 1822, 1855, and 1872, and have also 
 been described in the case of many other volcanoes. In 
 fig. 26 (p. 101 ) we have given representations of a group 
 of such irregular scoria-cones which was observed by 
 Schmidt on the Vesuvian lava of 1855. It will be seen 
 from this drawing that there is scarcely any limit to the 
 steepness of the sides of such scoria-heaps, in which 
 the materials are in an imperfectly solidified condition 
 when they reach the ground. 
 
 But in the majority of cases, the scoriae ejected from 
 volcanic vents are thrown to a great height, and are in 
 a more or less perfectly solidified condition when they 
 fall to the ground again. In such cases the falling 
 fragments obey the ordinary mechanical laws of sub- 
 stances, rolling and sliding over one another, till they 
 acquire a slope which varies according to the size and 
 form of the fragments. In this way the great conical 
 mounds are formed which are known as ' cinder-cones,' 
 or more properly as ' scoria-cones.' Scoria-cones 
 usually vary in the slope of their sides from 35 to 
 
154 VOLCANOES. 
 
 40, and may differ in size from mere monticules to 
 hills a thousand feet or more in height. Scoria-cones 
 of this character abound in many volcanic districts, as the 
 Auvergne, where they may be numbered by thousands. 
 The materials forming such scoria- cones vary in size 
 from that of a nut to masses as large as a man's head, 
 and fragments of even larger dimensions are by no 
 means uncommon. 
 
 When the lava in a volcanic vent is perfectly glassy, 
 instead of being partially crystalline in structure, we 
 find not scoriae but pumice ejected. In such cases, as 
 in the Lipari Islands for example, we see cones entirely 
 built up of pumice. Such pumice-cones resemble in 
 the angle of their slope (see fig. 41, facing p. 124), 
 the ordinary scoria-cones, but are of a brilliant white 
 colour, appearing as if covered with snow. 
 
 Ordinary scoriae are usually of a black colour when 
 first ejected, but after a short time the black oxide of 
 iron (magnetite) which they contain, attracts the oxygen 
 of the air and moisture, and assumes the reddish-brown 
 colour of iron-rust. Under such circumstances the 
 heaps of black material gradually acquire the red- 
 brown colour which is characteristic of so many of the 
 scoria-cones around Etna,, and in the Auvergne and the 
 Eifel. The moisture of the air, and the rain falling 
 upon these loose cindery heaps, cause them to decom- 
 pose upon their surfaces ; the action is facilitated by 
 the growth of the lower forms of vegetation, such as 
 mosses and lichens, and thus at last a soil is produced 
 
PKESEEVATION OF SCOKIA-CONES. 155 
 
 on the surfaces of these conical piles of loose materials 
 which may support an abundant vegetation. Cinder- or 
 scoria-cones are not uncommonly found retaining in 
 a most perfect manner their regular, conical form, the 
 lips of their craters being sharp and unbroken as if 
 the cone were formed but yesterday, while their slopes 
 may nevertheless be covered with a rich soil support- 
 ing abundant grass and forest-trees. It may at first 
 sight seem difficult to understand how a loose mass of 
 scoriae could have so long withstood the action of the 
 rain and floods, retaining so perfectly its even slopes 
 and sharp ridges. A little consideration will, however, 
 convince us that it is the very loose and pervious nature 
 of the materials of which scoria-cones are composed, 
 which tends to their perfect preservation. The rain at 
 once sinks into their mass, before it has time to form 
 rivulets and streams which would wear away their sur- 
 faces and destroy the regularity of their outlines. 
 
 Scoria- and pumice-cones are frequently found to 
 be acted upon by acid vapours to such an extent that 
 the whole of the materials is reduced to a white pul- 
 verulent mass. In these cases the oxides of iron arid 
 the alkalis have united with the sulphuric or hydro- 
 chloric or carbonic acids, the compounds being carried 
 away in solution by the rain-water falling on the mass ; 
 the materials left are silica, the hydrated silicate of 
 alumina, and hydrated sulphate of lime (gypsum), all of 
 which are of a white colour. 
 
 Cinder- or scoria-cones, and pumice-cones, are often 
 
156 VOLCANOES. 
 
 found raised by the action of winds to a greater eleva- 
 tion on one side than the other, in the manner already 
 described. One side of the cone is often seen to be 
 more or less completely swept away by an outwelling 
 stream of lava, and thus breached cones are formed 
 (see fig. 40, p. 123). Not unfrequently we find a num- 
 ber of Cones which are united more or less completely 
 at their bases, as in Vulcanello (fig. 6, p. 43), the 
 several vents being so near together that their ejec- 
 tions have mingled with one another. Cones composed 
 entirely of fragmentary materials often show an ap- 
 proach to the beautifully curved slopes which we have 
 described as being so characteristic of volcanoes, as 
 may be seen in fig. 41, facing p. 124. In the case of 
 scoria- and pumice-cones this curvature is probably due 
 to the rolling downwards and outwards of the larger 
 fragments. 
 
 We have already pointed out that with the scoriae 
 there are often ejected fragments torn from the sides 
 of the volcanic vents. Sometimes such fragments are 
 so numerous as to make up a considerable portion of 
 the mass of the volcanic cones. Thus in the Eifel we 
 find hills, of by no means insignificant size, completely 
 built up of small scoriae and broken fragments of slate 
 torn from the rocks through which the volcanic fissures 
 have been opened. Occasionally we see that few or 
 no scoriae have been ejected, and the volcanic vents are 
 surrounded simply by heaps of burnt slate. 
 
 The smaller fragmentary materials ejected from 
 
CHARACTERS OF TUFF-CONES. 157 
 
 volcanic vents such as lapilli and dust rest in heaps, 
 having a different angle of slope from those formed by 
 scoriae. In many cases, as we have seen, such finely- 
 divided materials descend in the condition of mud, 
 which flows evenly over the surface of the growing 
 cone and consolidates in beds of very regularly stratified 
 < tufa ' or ' tuff.' 
 
 The ' tuff-cones ' thus formed differ in many im- 
 portant respects from the scoria-cones already described. 
 The slope of their sides varies from 15 to 30, and 
 is almost always considerably less than in scoria- and 
 pumice-cones. The tuff-cones undergo much more 
 rapid degradation from rain and moisture than do the 
 scoria-cones ; for, though the materials of the former 
 6 set,' as we have seen, into a substance of considerable 
 hardness, yet this substance, being much less pervious 
 to water than the loose scoriae heaps, permits of the 
 formation of surface-streams which furrow and wear 
 away the sides of the cones. Sometimes the sides of 
 the crater are found to be almost wholly removed by 
 atmospheric denudation, and only a shallow depression 
 is found occupying the site of the crater ; such a case 
 is represented in fig. 59. We not unfrequently find 
 the whole slopes of such cones to be traversed by a 
 series of radiating grooves passing from the summit to 
 the base of the mountains, these channels being formed 
 by water, which has collected into streams, flowing 
 down the slopes of the mountains. The volcanic cone, 
 under these circumstances, frequently presents the 
 
158 
 
 VOLCANOES. 
 
 appearance of a partially opened umbrella. Owing to 
 the impervious character of the materials composing 
 tuff-cones, their craters are frequently found to be 
 occupied by lakes. 
 
 Tufas have usually a white or yellowish -brown 
 colour, and these are the colours exhibited by the 
 cones composed of this material before they become 
 covered by vegetation. Tufas scoriae, and lavas usually 
 
 FIG. 59. SUMMIT OF THE VOLCANO OF MONTE SANT' ANGELO IN LIPARI 
 EXHIBITING A CRATER WITH WALLS WORN DOWN BY DENUDATION. 
 
 crumble down to form a very rich soil, and many of the 
 choicest wines are produced from grapes grown on the 
 fertile slopes of volcanic mountains. When, however, 
 as not unfrequently happens, the materials are finely 
 divided and incoherent, they are so easily driven about 
 by the winds that cultivation of -any kind is rendered 
 almost impossible. In the Islands of Stromboli and 
 Vulcano the gardens have to be surrounded by high 
 fences to prevent them from being overwhelmed by the 
 ever-shifting masses of volcanic sand. 
 
CHARACTERS OF LAVA-CONES. 159 
 
 There are some cones which are composed in part 
 of scorise and in part of tufa. Hence we are some- 
 times at a loss whether to .group them with the one 
 class of cones or the other. But in the majority of 
 cases, scoria- and tuff-cones present the sufficiently 
 well-marked and distinctive characters which we have 
 described. 
 
 Lava-cones differ quite as greatly in their forms as 
 do the cones composed of fragmentary materials, the 
 variations being principally determined by the degree 
 of liquidity of the lavas. 
 
 We sometimes find that outwelling masses of lava, 
 when issuing in small quantities from a vent, accumu- 
 late in cauliflower- shaped masses, or sometimes in the 
 form of a column, or bottle. Professor J. I). Dana 
 describes many such fantastically-formed masses of 
 lava as being found in Hawaii, one of which is repre- 
 sented in fig. 25 (p. 100). 
 
 When the lava issues from the vent in great quan- 
 tities it tends to flow on all sides of it, and to build 
 up a great conical heap above the orifice. If the lava 
 be very liquid it flows to great distances, resting at a 
 very slight slope. Thus we find that the volcanoes of 
 Hawaii have been built up of successive ejections of 
 very liquid lava, which have formed cones having a 
 slope of only 6 to 8, but of such enormous dimensions 
 that the diameter of their bases is seventy miles and 
 their height 14,000 feet. 
 
 If, on the other hand, the lava be viscid, or very 
 
160 VOLCANOES. 
 
 imperfectly liquid in character, it tends to accumulate 
 immediately around the vent ; fresh ejections force 
 the first extruded matter outwards, in the manner 
 so well illustrated by Dr. Beyer's experiments, and at 
 last a more or less steep- sided bulbous mass is formed 
 over the vent. Such bulbous masses, composed of im- 
 perfectly fluid lavas, occur in many volcanic districts, 
 and constitute hills of considerable size. From the 
 
 FIG. 60. OUTLINES OF LAVA-CONES. 
 
 1. Mavma Loa, in Hawaii. Composed of very fluid lava. 
 
 2. The Scblossberg of Teplitz, Bohemia. Composed of very imperfectly fluid or 
 viscid lava. 
 
 tendency of matters thus extruded to choke up the 
 vents, however, these volcanoes composed of viscid 
 lavas cannot be expected to attain the vast dimensions 
 reached by some of those composed of very liquid 
 lavas. The difference in the forms of lava-cones com- 
 posed of very fluid or of somewhat viscid materials is 
 illustrated in fig. 58. When the interior of such steep- 
 sided volcanic mountains composed of viscid materials 
 is exposed by the action of denuding forces, the peculiar 
 internal structure we have described is displayed by 
 
CHAEACTEES OF COMPOSITE CONES. 161 
 
 them. In the Chodi-Berg of Hungary, a great bulbous 
 mass of andesitic rock, this endogenous structure is 
 admirably displayed. It is also well seen in the ex- 
 cavation of the hill of the Grand Sarcoui, a similar 
 mass, composed of altered trachyte, which has been 
 erupted in the midst of a scoria-cone in the Auvergne. 
 See fig. 44 (p. 126). 
 
 Most of the great volcanic mountains of the globe 
 belong to the class of ' composite cones,' and are built 
 up by alternate ejections of fluid lava and fragmentary 
 materials. The slope of the sides in such composite 
 cones is subject to a wide range of variation, being 
 determined in part by the degree of liquidity of the 
 lavas, in part by the nature of the fragmentary mate- 
 rials ejected, and in part by the proportions which the 
 fragmentary and lava-ejections bear to one another. 
 
 But there is another set of causes which tends to 
 modify the form and character of these composite, 
 volcanic cones. As we have already pointed out, the 
 sides of such cones are liable to be rent asunder from 
 time to time, and the fissures so produced are injected 
 with masses of liquid lava from below. These fissures, 
 rent in the sides of volcanic cones, often reach the 
 surface and eruptive action takes place, giving rise to 
 the formation of a cone, or series of cones, upon the 
 line of the fissure (fig. 61). Such small cones thrown 
 up on the flanks of a great volcanic mountain are 
 known as ' parasitic cones ' ; though subordinate to the 
 great mountain mass, they may be in themselves of 
 
162 
 
 VOLCANOES. 
 
 considerable dimensions. Among the hundreds of 
 parasitic cones which stud the flanks of Etna, there 
 are some which are nearly 800 feet in height. 
 
 FIG. 61. DIAGRAM ILLUSTRATING THE FORMATION OF PARASITIC CONES 
 ALONG LINES OF FISSURE FORMED ON THE FLANKS OF A GREAT 
 VOLCANIC MOUNTAIN. 
 
 The building up of parasitic cones upon the flanks 
 of a volcanic mountain tends, of course, to destroy its 
 regular conical form. This may be well seen in Etna, 
 
 FIG. 62. OUTLINE OF ETNA, AS SEEN FROM CATANIA. 
 
 which, by the accumulation of materials upon its flanks, 
 has become a remarkably 6 round-shouldered ' moun- 
 tain. (See figs. 62 and 63,) At the same time it must 
 
FORMATION OF PARASITIC CONES. 
 
 163 
 
 rrrft 
 
 FIG. 63. OUTLINE OF ETNA, AS SEEN FROM THE VAL DEL BOVE. 
 
 be remembered that materials erupted from the central 
 vent tend to fill up the hollows between these parasitic 
 
 FIG. 64. PLAN OF THE VOLCANO FORMING THE ISLAND OF ISCHIA. 
 
 a, a^t. The semi-circular crater ring of Epomeo 
 6, c, d. Lava-currents which have flowed from the principal crater. 
 e,f, g, h. Plateaux form n by ancient lava-currents. 
 k. Montagnone \ 
 I. Monte Rotaro 
 
 m. Monte Tabor I Parasitic cones and craters on the slopes of the 
 7i. Castiglione { mountain. 
 
 o. Lago di Bagno. 
 p. The Cremate. 
 
 r. Lava-stream of the Arso, which flowed from the Cremate in 1301. 
 X, X, x. Raised beaches on the shores of the island, showing that it has 
 recently undergone elevation. 
 
164 
 
 VOLCANOES. 
 
 cones, and thus to restore to the mountain its regularly 
 conical form. 
 
 The Island of Ischia is a good example of a great 
 volcanic cone the flanks of which are covered with nu- 
 merous small parasitic cones. While the great central 
 volcano has evidently been long extinct, and one side of 
 its crater-wall is completely broken down, some of the 
 small parasitic cones around its base have been formed 
 within the historical period one of them as recently 
 as the year 1301. Fig. 64 is a plan of the Island of 
 
 FIG. 65. A PRIMARY PARASITIC CONE WITH A SECONDARY ONE AT ITS 
 BASE ISCHIA. 
 
 a. Monte Rotaro. b. Monte Tabor. c. Lava-stream flowing from the latter. 
 
 Ischia, showing the numerous parasitic cones scat- 
 tered over the slopes of the principal cone. 
 
 In one case we find that a parasitic cone, the Monte 
 Eotaro, has itself a similar smaller cone, which is para- 
 sitic to it, at its foot ; this secondary parasitic cone 
 gives off a small lava-stream of trachyte, which has 
 flowed down to the sea. (See fig. 65.) 
 
 Most great volcanic mountains exhibit a tendency 
 towards a subsidence of their central portions, which 
 may take place either during or subsequently to their 
 
UNIVERSITY 
 
 SUBSIDENCE BENEATH VOLCANIC VENTS. 
 
 165 
 
 period of activity. When we examine the strata upon 
 which a volcano has been built up, but which are now 
 exposed to our study by denuding forces, we usually 
 
 FIG. 66. SCORIA-CONE NEAR AUCKLAND, NEW ZEALAND, WITH A LAVA- 
 CURRENT FLOWING FROM IT. 
 
 The strata beneath the volcanic cone are exposed in the sea cliff, and exhibit proofs 
 of depression having taken place. 
 
 find that they incline towards the centre of the erup- 
 tive activity. (See figs 66 and 67.) Two causes may 
 
 Predazzo 
 
 FIG. 67. SECTION OF ROCKS BELOW THE ANCIENT TRIASSIC VOLCANO 
 OF PREDAZZO IN THE TYROL. 
 
 The position of the strata a be, etc., indicates a central subsidence. 
 
 contribute to bring about this result. In the first 
 instance, we must remark that the piling up of ma- 
 terials around the volcanic vent causes the subjacent 
 
166 VOLCANOES. 
 
 strata to be subjected to a degree of pressure far in 
 excess of that which acts upon the surrounding rocks. 
 And secondly, it must be borne in mind that the con- 
 tinual removal of material from below the mountain 
 must tend to the production of hollows, into which 
 the overlying strata will sink. The effect of this cen- 
 tral subsidence is to give to the flanks of volcanic 
 cones those beautifully curved outlines which constitute 
 so striking a feature in Vesuvius (see fig. 1 7, p. 87), 
 Fusiyama (see fig. 77, No. 1, facing p. 178), and many 
 other volcanic mountains. 
 
 There seems, at first sight, to be scarcely any limit 
 to the dimensions which these great composite volcanic 
 cones may attain : the lateral eruptions tending to 
 enlarge the area of the base of the mountain, and, 
 by the injection of the fissures, to knit together and 
 strengthen its structure, while the central eruptions 
 continually increase the elevation of the mass. Great, 
 however, as is the force which is concerned in the pro- 
 duction of our terrestrial volcanoes, it has its limits ; 
 and, at last, the piling up of materials will have gone 
 on to such an extent, that the active forces beneath 
 the volcano are no longer competent either to raise 
 materials to the elevated summit of the mountain or 
 to tear asunder its strengthened and fortified flanks. 
 Under these circumstances, the volcanic forces, if they 
 have not already exhausted themselves, will be com- 
 pelled to find weak places in the district surrounding 
 the volcano, at which fissures may be produced and the 
 phenomena of eruption displayed. 
 
SHIFTING OF VOLCANIC FOCI. 167 
 
 Some volcanic cones exhibit evidence that during 
 the series of eruptions by which they have gradually 
 been built up, the centre of volcanic action has shifted 
 to another point within the mountain. Thus Lyell has 
 shown, in the case of Etna, that during the earlier 
 periods in the history of the mountain the pi] ing up 
 of materials went on around a centre which is now 
 situated at a distance of nearly four miles from the 
 present focus of eruptive activity. Some of our old 
 British volcanoes, of which the denuded wrecks exist 
 in the Western Isles of Scotland, show similar evidence 
 of a shifting of the axis of eruption. 
 
 One of the most conspicuous features of a volcanic 
 cone is the great depression or crater found at its 
 summit. In describing the internal structure of vol- 
 canic cones, we have seen how these craters are pro- 
 duced and acquire their inverted conical form, by the 
 slipping and rolling back of materials towards the 
 centre of eruptive action. 
 
 Almost all volcanic cones exhibit craters, but in 
 those which are formed entirely by the outwelling of 
 viscid lavas the central depression is often slight and 
 inconspicuous, and occasionally altogether wanting. It 
 frequently happens, however, that eruptive action has 
 ceased at the centre of a volcano, and its summit- 
 crater may by denudation be entirely destroyed, while 
 new and active craters are formed upon its flanks. 
 Stromboli furnishes us with an admirable example of 
 this kind (see fig. 1, facing p. 10). Other volcanoes 
 
168 
 
 VOLCANOES. 
 
 may exhibit several craters, one at the summit of the 
 mountain and others upon its flanks. Of this we find 
 a good example in Vulcano (fig 6, p. 43). 
 
 When a volcano has been built up by regular and 
 continuous eruptions from the same volcanic vent, the 
 size of the crater remains the same, while the volcano 
 continually grows in height and in the diameter of its 
 
 .Snow-lin< 
 
 FIG. 68. COTOPAXI (19,GOO FEET), AS SEEN FROM A DISTANCE OF 
 
 NINETY MILES 
 
 base. The size of the crater will be determined by the 
 eruptive force at the volcanic centre, the size of the 
 mountain by the duration of the volcanic activity and 
 the quantity of material ejected. In the earliest stage 
 of its history, such a volcano will resemble Monte 
 Nuovo, which has a crater reaching down almost to the 
 base of the mountain ; in the later stages of its history, 
 such a volcano will resemble Cotopaxi (fig. 68) and 
 Citlaltepetl (fig. 69), in which the crater, though of far 
 
ORIGIN OF VOLCANIC CRATERS. 
 
 greater absolute dimensions than that of Monte Nuovo, 
 bears but a small proportion to the vast cone at the 
 summit of which it is situated. 
 
 In the great majority of volcanoes, however, erup- 
 tive action does not go on by any means regularly and 
 continuously, but terrible paroxysmal outbursts occur, 
 
 FIG. 69. CITLALTEPETL, OR THE Pic D'ORIZABA, IN MEXICO (17,370 
 FEET), AS SEEN FROM THE FOREST OF XALAPA. 
 
 which suddenly enlarge the dimensions of the crater to 
 an enormous extent. 
 
 In the year 1772, there occurred a volcanic eruption 
 in the Island of Java, which is perhaps the most violent 
 and terrible that has happened within the historical 
 period. A lofty volcanic cone, called Papandayang, 
 9,000 feet high, burst into eruption, and, in a single 
 night. 30,000,000.000 cubic feet of materials were 
 
170 VOLCANOES. 
 
 thrown into the atmosphere, falling upon the country 
 around the mountain, where no less than forty villages 
 were buried. After the eruption, the volcano was found 
 to have been reduced in height from 9,000 to 5,000 
 feet, and to present a vast crater in its midst, which 
 had been formed by the ejection of the enormous mass 
 of materials. 
 
 Many similar cases might be cited of the removal 
 of a great part of a mountain-mass by a sudden, 
 paroxysmal outburst. In some cases, indeed, the whole 
 mass of a mountain has been blown away during a 
 terrific eruption, and the site of the mountain is now 
 occupied by a lake. This is said to have been the case 
 with the Island of Timor, where an active volcano, 
 which was visible from a distance of 300 miles at sea, 
 has entirely disappeared. 
 
 The removal of the central portion of great vol- 
 canic mountains by explosive action, gives rise to the 
 formation of those vast, circular, crater-rings of which 
 such remarkable examples occur in many volcanic dis- 
 tricts. These crater-rings present a wall with an outer 
 slope agreeing with that of the volcanic cone of which 
 they originally formed a part, but with steep inner 
 cliffs, which exhibit a good section of the beds of tuff, 
 ash, and Java with the intersecting dykes of which the 
 original volcano was built up. Near Naples, one of 
 these crater-rings, with sloping outer sides and steep 
 inner ones, is employed to form the royal game pre- 
 serve of Astroni, the only entrance to the crater being 
 closed by gates. 
 
FORMATION OF CEATEK-LAKKS. 
 
 171 
 
 As these crater-rings are usually composed of mate- 
 rials more or less impervious to water, they often become 
 the site of lakes. The beautiful circular lake of Laach, 
 in the Khine Provinces, with the numerous similar 
 examples of Central Italy Albano, Nemi, Bracciano, 
 and Bolsena the lakes of the Campi Phlegraei (Agnano, 
 Avernus, &c.), and some similar lakes in the Auvergne, 
 
 FIG. 70. LAC PAVEN, IN THE AUVERGNE. 
 
 s. Scoriae. b. 
 
 may be adduced as examples of crater-rings which have 
 become the site of lakes. 
 
 One of the most beautiful of the crater-lakes in the 
 Auvergne is Lac Paven (fig. 70), which lies at the foot of 
 a scoria-cone, Mont Chalme, and is itself surrounded by 
 masses of ejected materials. The crater-lake of Bagno, 
 in Ischia (fig. 71), has had a channel cut between it 
 and the sea, so that it serves as a natural harbour. 
 The lake of Grustavila, in Mexico (fig. 72), is an ex- 
 ample of a crater-lake on a much larger scale. 
 
172 
 
 VOLCANOES. 
 
 In many of these crater-rings the diameter of the 
 circular space enclosed by them is often very great 
 indeed as compared with the height of the walls. 
 
 FIG. 71. THE CRATER-LAKE CALLED LAGO DEL BAGNO, IN ISCIIIA, 
 
 CONVERTED INTO A HARBOUR. 
 
 Two of the largest crater-rings in the world are 
 found in Central Italy, and are both occupied by lakes, 
 the circular forms of which must strike every observer. 
 
 FIG. 72. LAKE OF GUSTAVILA, IN MEXICO. 
 (The terraces round the lake have been artificially formed.) 
 
 The Lago di Bracciano, which lies to the north-west of 
 Eome, is a circular lake six and a half miles in diameter, 
 surrounded by hills which at their highest point rise to 
 the height of 1,486 feet above the sea, while the surface 
 
DIMENSIONS OF CRATEK-LAKES. 173 
 
 of the waters of the lake is 540 feet above the sea- 
 level. The Lago di Bolsena is somewhat less perfectly 
 circular in outline than the Lago di Bracciano ; it has a 
 length from north to south of ten-and-a-quarter miles 
 and a breadth from east to west of nine miles ; the surface 
 of the waters of this lake is 962 feet above that of 
 the waters of the Mediterranean. The lake of Bolsena, 
 like that of Bracciano, is surrounded by hills composed 
 of volcanic materials ; the highest points of this ring 
 of hills rise to elevations of 684, 780, and 985 feet 
 respectively above the waters of the lake. 
 
 In these great circular lakes of Bolsena and Brac- 
 ciano, as well as in the smaller ones of Albano, Nemi, 
 and the lakes of Frascati in the same district, the va^-t 
 circular spaces enclosed by them, the gradual outer 
 slope of the ring, and the inner precipices which bound 
 the lake, all afford evidence of the explosive action to 
 which they owe their origin. 
 
 But while the vast crater-rings we have mentioned 
 are frequently found to be occupied by lakes, there are 
 many other similar crater -rings which remain dry, either 
 from the materials of which they are composed being 
 of more pervious character, or from rivers having cut a 
 channel through the walls of the crater, in this way 
 draining off its waters. 
 
 Thus in the Campi Phlegraei, while we have the 
 craters of Agnano and Avernus forming complete cir- 
 cular lakes, Astroni has only a few insignificant lakelets 
 on its floor, and the Pianura, the Piano di Quarto, 
 
174 VOLCANOES. 
 
 which have each a diameter of three or four miles, with 
 many others, remain perfectly dry. In the vicinity of 
 the great crater-lakes of Central Italy we find the crater- 
 ring of the Vallariecia, which has evidently once been 
 a lake but is now drained, its floor being covered with 
 villages and vineyards. 
 
 A comparison of these vast crater-rings leads us to the 
 conclusion that in the majority of cases, if not in every in- 
 stance, they are composed almost entirely of volcanic tuff 
 and dust. In the case of the more solidly-built composite 
 volcanic cones, the volcanic forces, as we have seen, 
 produce fissures in the mass, and along these fissures 
 parasitic cones are thrown up, the tension of the mass 
 of imprisoned vapours below the mountain being thus 
 from time to time relieved. But in the case of a vol- 
 canic cone composed of loose fragmentary materials, such 
 temporary relief is impossible. The cracks, as soon as 
 they originate, will be filled up and choked by the fall- 
 ing in of materials from above and at their sides. In 
 this way the eruptive action will be continually re- 
 pressed, till at last the imprisoned vapours acquire such 
 a high state of tension that the outburst, when it occurs, 
 is of the most terrible character, and the whole central 
 mass of the volcano is blown into the air. It may often 
 seem surprising that the ejection of such vast masses of 
 material from the centre of a volcanic cone does not 
 effect more in the way of raising the height of the 
 crater-walls. But it must be remembered that, in the 
 case of craters of such vast area, the majority of the 
 
CRATER-KINGS SURROUNDING CONES. 
 
 175 
 
 ejected materials must fall back again within its cir- 
 cumference. By repeated ejections these materials will 
 at last be reduced to such an extreme state of comminu- 
 tion that they can be borne away by the winds, and 
 spread over the country to the distance of hundreds 
 or thousands of miles. After great volcanic outbursts 
 enormous areas are thus found covered with fine vol- 
 canic dust to the depth of many inches or feet. 
 
 FIG. 73. PEAK OF TENERIFFE IN THE CANARY ISLANDS (12,182 FT'.), 
 SURROUNDED BY GREAT CRATER-RINGS. 
 
 Sometimes, as in the case of the Lago di Bracciano, 
 the eruptive forces appear to have entirely exhausted 
 themselves in the prodigious outburst by which the 
 great crater was produced. But in other cases, as in 
 that of the Lago di Bolsena, the eruptive action was 
 resumed at a later date, and small tuff-cones were 
 thrown up upon the floor of the crater; these now 
 rise as islands above the surface of the lake. In other 
 cases, again, the eruptive action was resumed after the 
 
176 
 
 VOLCANOES. 
 
 formation of the great crater-ring, with such effect that 
 bulky volcanic cones were built up in the midst of the 
 
 FIG. 74. THE VOLCANO OF BOURBON, RISING IN THE MIDST OF A 
 CRATER-RING FOUR MILES IN DIAMETER. 
 
 crater- ring which surrounds them like a vast wall ; ex- 
 amples of this are exhibited in the extinct volcanoes 
 
 FIG. 75. THE VOLCANO OF BOURBON, AS SEEN FROM ANOTHER POINT OF 
 
 VIEW, WITH THREE CONCENTRIC CRATER-RINGS ENCIRCLING ITS BASE. 
 
 of Eocca Monfma and Monte Albano. Some of the 
 grandest volcanoes of the globe, such as Teneriffe (fig. 
 73), the volcanoes of Mauritius and Bourbon (figs. 74 
 
BASALTIC CONES IN TRACHYTIC CRATER- KINGS. 177 
 
 and 75), and many others that might be cited, are thus 
 found to be surrounded by vast crater-rings. Vesuvius 
 itself is surrounded by the crater-ring of Somma (fig. 
 76). 
 
 This formation of cone within crater, often many 
 times repeated, is very characteristic of volcanoes. The 
 craters mark sudden and violent paroxysmal outbursts, 
 the cones are the result of more moderate but long- 
 continued ejection. Sometimes, as at Vesuvius in 1767 
 
 FIG. 76. VESUVIUS, AS SEEN FROM SORRENTO, HALF ENCIRCLED BY 
 THE CRATEK-RING OF SOMMA. 
 
 (fig. 15, p. 85), we find a nest of craters and cones which 
 very strikingly exemplifies this kind of action. 
 
 We shall point out, hereafter, that at most volcanic 
 centres the ejection of trachytic lavas precedes that of-. N * 
 the basaltic lavas. Now it is these trachytic lavas which 
 principally give rise to the formation of the light lapilli 
 of which tuff-cones are formed. Hence it is that we 
 so frequently find, as in the case of Vesuvius, Rocca- 
 Monfina, and many other volcanoes, that a great crater-* 
 ring, largely composed of tuffs, encloses a cone built 
 up of more basic lavas. 
 9 
 
178 - VOLCANOES. 
 
 In fig. 77 we have shown by a series of outline 
 sections the various forms assumed by volcanoes in 
 consequence of the different kinds of eruptive action 
 going on in them : 
 
 1. Is an outline of Fusiyama, an almost perfect 
 cone, with a small crater at its summit. The sides of 
 this volcano admirably illustrate the beautiful double 
 curves characteristic of volcanic cones. 
 
 2. Hverfjall in Iceland, a volcanic cone with a large 
 crater, reaching almost to its base. 
 
 3. The crater-lake of Bracciano, in which the area 
 of the crater is out of all proportion to the height of 
 the crater-walls. 
 
 4. Rocca-Monfina, in Southern Italy, a tuff-cone of 
 large dimensions, in the midst of which an andesitic 
 lava-cone has been built up. 
 
 5. Teneriffe, in the Canary Islands, in which a per- 
 fect volcanic cone has been built up in the centre of an 
 encircling crater-ring. 
 
 6. Yulcano, in the Lipari Islands, in which, by the 
 shifting of the centre of volcanic activity along a line 
 of fissure, a series of overlapping volcanic cones has 
 been produced. 
 
 While speaking of the varieties of form assumed by 
 volcanic cones and craters, we must not forget to notice 
 the effects which are produced by denuding forces upon 
 them. In the case of submarine volcanoes, like the 
 celebrated island called by the English Graham Isle, by 
 the French Isle Julie, and by the Germans the Insel 
 
FIG. 77. OUTLINES OF VARIOUS VOLCANOES, ILLUSTRATING THE 
 DIFFERENT RELATIONS OF THE CRATERS TO CONES. 
 
SUBMAKINE VOLCANOES. 179 
 
 Ferdinandez (fig. 78), which was thrown up off the 
 coast of Sicily in 1831, it was evident that volcanic 
 outbursts taking place at some depth below the level 
 of the sea gradually piled up a cone of scoriae with a 
 crater in its midst. By constant accessions to its 
 mass, this scoria-cone was eventually raised above 
 the sea-level, but the action of the waves upon the 
 
 FIG. 78. ISLAND THROWN UP IN THE MEDITERRANEAN SEA IN JULY 
 
 AND AUGUST 1831. 
 
 (The view was taken in the month of September, after the sides of the 
 crater had been washed away by the waves.) 
 
 loose materials soon destroyed the crater- walls and 
 eventually reduced the island to a shoal. It is evident 
 that in all cases in which eruptions take place beneath 
 the sea-level, and the loose materials are exposed during 
 their accumulation to the beating of the sea-waves, the 
 form of the volcanic cone so produced will be greatly 
 modified by the interaction of the two sets of opposed 
 causes, the eruptive forces from below and the distri- 
 butive action of the sea-waves. 
 
180 VOLCANOES. 
 
 Craters when once formed are often rent across, 
 along the line of the fissure above which they are 
 thrown up. Thus the crater of Vesuvius was in 1872 
 rent completely asunder on one side, so that it was 
 possible to climb through the fissure thus produced and 
 reach the bottom of the crater. Streams flowing down 
 the sides of the crater, and escaping through such a rent, 
 may in the end greatly modify the form and disguise 
 the characters of a volcanic crater. Of this kind of 
 action we have a striking example in the Val del Bove 
 of Etna. 
 
 Volcanoes, as we shall point out in the sequel, are 
 after their extinction frequently submerged beneath the 
 waters of the ocean. The sea entering the craters, 
 eats back their cliff-like sides and enlarges their areas. 
 Such denuded craters are called ' calderas,' the channels 
 into them ' barrancos.' 
 
 Sometimes the action of the waves upon a partially 
 submerged volcano has led to the cutting back of its 
 slopes into steep cliffs, at the same time that the crater- 
 ring is enlarged. In such cases we have left a more 
 or less complete rocky ring, composed of alternating 
 lavas and fragmentary materials. Of such a ruined 
 crater-ring, the Island of St. Paul in the South Atlantic 
 affords an admirable example. 
 
 When the action of denudation has gone still far- 
 ther, all the lavas and tuffs composing the cone may be 
 completely removed and nothing left but masses of the 
 hard and highly-crystalline rocks which have cooled 
 
FORMATION OF MUD- VOLCANOES. 181 
 
 down slowly in the heart of the volcano. An example 
 of this kind is afforded to us by St. Kilda, the remotest 
 member of the British Archipelago. 
 
 But although the majority of volcanic craters are 
 clearly formed by explosive action, there are some 
 craters, like those of Kilauea in Hawaii, which probably 
 owe their origin to quite a different set of causes. In 
 this case the explosive action at the vent is but slight, 
 and the crater, which is of very irregular form, appears 
 to have originated in a fissure, which has been slowly 
 enlarged by the liquid lavas encroaching upon and 
 eating away its sides. Such craters as these, however, 
 appear to be comparatively rare. 
 
 Besides the great volcanic mountains composed < f 
 lava, scoriae, tuff and ash, there are other structures 
 which are formed around volcanic vents even when 
 these do not eject molten rock-masses. The water 
 which issues in these cases either as steam or in a 
 more or less highly heated condition frequently carries 
 materials in suspension or solution, and these some- 
 times accumulate in considerable quantities around the 
 vent. 
 
 When fissures are formed in the midst of loose 
 argillaceous materials, such as are frequently produced 
 by the decomposition of volcanic rocks, the waters 
 which issue through them are sometimes so charged 
 with muddy matter that this accumulates to form cones 
 having all the general characters of volcanic mountains, 
 and which occasionally rise to the height of 250 feet. 
 
182 VOLCANOES. 
 
 The gases and vapours which issue from these i mud- vol- 
 canoes ' are those which are known to be emitted from 
 volcanic vents at which the action going on is not very 
 intense. Daubeny and others have suggested that 
 these mud-volcanoes may be the result of actions which 
 have little or no analogy with those which take place 
 at ordinary volcanic vents, and that the combustion of 
 Subterranean beds of sulphur and similar causes would 
 be quite competent to their production. But inas- 
 much as these mud-volcanoes are almost always situated 
 in regions in which the more powerful volcanic action 
 has only recently died away, and the gases and vapours 
 emitted by them are very similar in character to those 
 which issue from volcanoes, there does not appear to 
 be any good reason for doubting that they should be 
 classed as truly volcanic phenomena. 
 
 Mud-volcanoes are found in Northern Italy near Mo- 
 dena, in Sicily near Girgenti, on the shores of the Sea of 
 Azof and the Caspian, in Central America, and in other 
 parts of the globe. The gas frequently escapes from 
 them with such violence that mud is thrown into the 
 air to the height of several hundreds of feet. Some- 
 times this gas is inflammable, consisting of sulphuretted 
 hydrogen, hydrogen, or some hydrocarbons, and these 
 gases occasionally take fire, so that true flames issue 
 from these mud-volcanoes. In other cases the mud- 
 volcanoes appear to be formed by either hot or cold 
 springs containing large quantities of suspended mate- 
 rials, and the liquid mud issues from the vent without 
 any violent eruptive action. 
 
FORMATION OF SINTER-CONES. 183 
 
 The soluble materials which waters issuing from 
 volcanic vents deposit on their sides are chiefly silica 
 and carbonate of lime. 
 
 Hot springs, whether intermittent or constant, 
 often contain large quantities of silica in solution. 
 The solution of this silica is effected, at the moment of 
 its separation from combination with the alkali or 
 
 FIG. 79. SINTER-CONES SURROUNDING THE ORIFICES OF GEYSERS. 
 
 1. Basin of the Great Geyser, Iceland. 2. Hot spring cone. 3. Old Faithful. 
 
 4. The Giant Geyser. 5. Liberty Cap. (2, 3, 4 and 5 are in the 
 
 Yellowstone Park district of the Rocky Mountains.) 
 
 alkaline earths, during the decomposition of volcanic 
 rocks, and is favoured by the presence of alkaline car- 
 bonates in the water, and the high temperature and 
 the pressure under which it exists in the subterranean 
 regions. When the water reaches the surface and, 
 being relieved from pressure, begins to cool down the 
 silica is deposited. By this deposited silica the basins 
 around the geysers of Iceland are formed. Sometimes 
 conical structures are built up around the vents of hot 
 springs by the deposition of silica from their waters. 
 
184 VOLCANOES. 
 
 Examples of this kind abound in the National Park of 
 Colorado, where they have received fanciful names, such 
 as the Beehive, Liberty Cap, &c. This deposited silica 
 is known to geologists as sinter. The forms of some 
 of the structures which surround the orifices of geysers 
 is shown in fig. 79. The ' Liberty Cap ' is an extinct 
 geyser-cone fifty feet high and twenty feet in diameter. 
 
 Hot and cold springs rising in volcanic regions are 
 often highly charged with carbonic acid, and in passing 
 through calcareous rocks dissolve large quantities of 
 carbonate of lime. Upon exposure to the atmosphere, 
 the free carbonic acid escapes and the carbonate of lime 
 is deposited in the form known as ' travertine.' Such 
 springs occur in great numbers in many volcanic re- 
 gions. In the Auvergne great rock-masses occur 
 formed of carbonate of lime deposited from a state of 
 solution and taking the form of natural aqueducts and 
 bridges. In Carlsbad the numerous hot springs have 
 deposited masses of pisolitic rock (Strud el stein) which 
 have filled up the whole bottom of the valley, and upon 
 these deposits the town itself is mainly built. In Cen- 
 tral Italy the deposits of travertine formed by cal- 
 careous springs are of enormous extent and thickness : 
 St. Peter's and all the principal buildings of Kome 
 being constructed of this travertine or ' Tibur-stone.' 
 
 When springs charged with silica or carbonate of 
 lime rise upon the slope of a hill composed of loose 
 volcanic materials, they give rise to the remarkable 
 structures known as sinter- and travertine-terraces (see 
 
FORMATION OF SINTER-TERRACES. 185 
 
 fig. 80). The water flowing downwards from the vent 
 forms a hard deposit upon the lower slope of the hill, 
 while the continual deposition of solid materials with- 
 in the vent tends to choke it up. As a new vent 
 cannot be forced by the waters through the hard rock 
 formed below, it is originated a little higher up. Thus 
 
 FIG. 80. DIAGRAM ILLUSTRATING THE MODE OF FORMATION OF TRAVER- 
 TINE AND SINTER TERRACES ON THE SIDES OF A HILL OF TUFF. 
 
 the site of the spring is gradually shifted farther and 
 farther back into the hill. As deposition takes place 
 along the surfaces over which this water flows, ter- 
 races are built up enclosing basins. Of structures of 
 this kind we have remarkable examples in the sinter- 
 terraces of Eotomahana in New Zealand and the tra- 
 vertine-terraces of the Gardiner's Eiver in the Yellow- 
 stone Park district of the Kocky Mountains. 
 
186 VOLCANOES. 
 
 CHAPTER VII. 
 
 THE SUCCESSION OF OPERATIONS TAKING PLACE AT 
 VOLCANIC CENTRES. 
 
 THAT a volcanic vent, when once established, may 
 display intense activity during enormous periods of 
 time, there cannot be the smallest reason for doubting ; 
 for the accumulation of materials around some existing 
 volcanic centres must certainly have been going on 
 during many thousands, perhaps millions, of years. 
 To us, whose periods of observation are so circum- 
 scribed, it may therefore at first sight appear a hopeless 
 task to trace the ' life-history of a volcano,' to discover 
 the stages of its development, and to indicate the 
 various episodes which have occurred during the long 
 periods it has been in existence. But when it is re- 
 membered that we have the opportunity of studying 
 and comparing hundreds of such volcanoes, exhibiting 
 every varying phase of their development, we shall 
 see that such an attempt is by no means so un- 
 promising as it at first sight appears .to be. In the 
 present chapter, we shall give an account of the results 
 which have already been obtained by inquiries directed 
 to this object. 
 
CYCLES OF VOLCANIC PHENOMENA. 187 
 
 There is not the smallest room for doubt that 
 during the past history of our globe, exhibitions of 
 subterranean energy have occurred at many different 
 parts of its surface. There is further evidence that 
 at the several sites where these displays of the vol- 
 canic forces have taken place, the succession of the 
 outbursts has run through a regular cycle, gradually 
 increasing in intensity to a maximum, and then as 
 gradually dying away. 
 
 A little consideration will show that the first portion 
 of this cycle of events is the one which it is most diffi- 
 cult to examine and study. The products of the earlier 
 and feeble displays of volcanic activity, at anyjparticu- 
 lar centre, are liable to be destroyed, or masked, during 
 the ejection of overwhelming masses of materials in 
 the later stages of its more matured energy. That the 
 feeble displays of volcanic force now exhibited in some 
 localities will gradually increase in intensity in the 
 future, and eventually reach the grandest stage of de- 
 . velopment, there can be no reason for doubting. But, 
 unfortunately, we are quite unable to discriminate 
 these feeble manifestations, which are the embryonic 
 stages in the development of grand exhibitions of the 
 volcanic forces, from slight outbursts which die away 
 and make no further sign. 
 
 From what has been proved concerning the true 
 nature of volcanic action, however, it is certain that 
 the first step towards the exhibition of such action, at 
 any particular locality, must be the production of an 
 
188 VOLCANOES. 
 
 aperture in the earth's crust. Only by means of such 
 an aperture can the vapours, gases, and rocky materials 
 reach the surface, and give rise to the phenomena 
 there displayed. There is reason to believe that all 
 such operations are really of the nature of fissures, or 
 cracks, which have been opened through the super- 
 jacent strata by the efforts of the repressed subter- 
 ranean forces. 
 
 Some recent writers have, it is true, endeavoured 
 to draw a distinction between what they call ' fissure- 
 eruptions,' and eruptions taking place from volcanic 
 cones. But all volcanic outbursts are truly 'fissure- 
 eruptions ' the subterranean materials finding their 
 way to the surface through great cracks, which, in a 
 more or less vertical position, traverse the overlying 
 rock-masses. It is true that in many cases portions of 
 these cracks soon get choked up, while other portions 
 become widened, and the volcanic energy is concen- 
 trated at such spots. Thus the materials ejected from 
 these fissures are usually emitted in greatest quantities 
 at one or more points along the fissure, and a single 
 great volcanic vent, or a row of smaller vents, is esta- 
 blished upon the line at which the fissure reaches the 
 surface. 
 
 We have seen that the amount of explosive action 
 taking place at different volcanic vents varies according 
 to the proportion of imprisoned water contained in the 
 lava. In the cases where there is much explosive 
 action, vast accumulations of scoriae, lapilli, and dust 
 
FOKMATION OF VOLCANIC FISSURES. 189 
 
 take place, and cones of great size are built up ; but 
 in those cases where the explosive action is small the 
 lavas flow quietly from the vent, and only small scoriae- 
 cones are thrown up, these being probably soon swept 
 away by the lava-currents themselves or by denuding 
 agencies. But both kinds of eruption have equal 
 claims to be called ' fissure-eruptions.' 
 
 In the expansive force of great masses of im- 
 prisoned vapour, we have a competent cause for the 
 production of the fissures through which volcanic out- 
 bursts take place. Such fissures are found traversing 
 the rocks lying above volcanic foci, and often extending 
 to distances of many miles, or even hundreds of miles, 
 from the centres of activity. Some of these cracks 
 are found to be injected with fused materials from 
 below, others have been more or less completely filled 
 with various minerals that have been volatilized, or 
 carried by superheated waters from the deeper regions 
 of the earth's crust. That many of the cracks thus 
 produced in the superjacent rocks, by the heaving 
 forces of imprisoned vapour seeking to escape, never 
 reached the surface, we have sufficient proof in many 
 mining regions. 
 
 If we now transfer our attention from the deeper 
 portions of the earth's crust to the surface, we can 
 well understand how the attempts of the imprisoned 
 vapours to force a passage for themselves through the 
 solid rock-masses would lead to shocks and jars among 
 the latter. Each of these shocks or jars would give 
 
190 VOLCANOES. 
 
 rise, in the surrounding portions of the earth's crust, 
 to those vibrations which we know as earthquakes. 
 The close connection between most earthquakes and 
 volcanic phenomena is a fact that does not admit of 
 the smallest doubt; and though it would be rash to 
 define all earthquakes as * uncompleted efforts to esta- 
 blish a volcano, 5 yet, in the efforts of the repressed 
 subterranean forces to find a vent by the production 
 of fissures in the overlying rock-masses, we have a 
 cause competent to the production of those shocks 
 which are transmitted to such enormous distances as 
 waves of elastic compression. 
 
 We have seen that the production of the fissure 
 upon which the small volcano of Monte Nuovo was 
 thrown up was preceded by a succession of earth- 
 quakes, which for a period of over two years terrified 
 the inhabitants of the district, and might have warned 
 them of the coming event. In the same manner, 
 doubtless, the period before the appearance of volcanic 
 phenomena in a new area would be marked by power- 
 ful subterranean disturbances within it, due to the 
 efforts of the imprisoned vapours to force for them- 
 selves a channel to the surface. 
 
 In the case of Monte Nuovo, we have seen that the 
 fissure, when first produced, emitted water at first in 
 a cold, then in a boiling condition and, finally, steam 
 and scorise. It is probable that through the first 
 cracks which reached the surface, during the heaving 
 of the subterranean forces, water, charged with car- 
 
MATURE OF FIRST EJECTIONS FROM FISSURES. 191 
 
 bonic acid, flowed abundantly, and that these cold 
 springs, charged with carbonic acid and carbonate of 
 lime, would be succeeded by others which were hot 
 and contained silica in solution. In Hungary, the 
 Western Isles of Scotland, and many other volcanic 
 districts, we find abundant evidence that, before the 
 eruption of lavas in the area, great masses of travertine 
 and siliceous sinter were formed by the action of cold 
 and hot springs. 
 
 As the volcanic action became more intense by the 
 more perfect opening of the fissures, the evolution of 
 carbonic acid gases would be succeeded by the appear- 
 ance of sulphurous acid, sulphuretted hydrogen, boracic 
 acid, and hydrochloric acid, which recent studies have 
 shown to be successively emitted from volcanic vents 
 as the temperature within them rises. At last lava or 
 molten rock becomes visible within the fissures, and 
 the ejection of the frothy masses scoriaa, pumice, lapilli 
 and dust commences, and this is sometimes succeeded 
 by the outflow of currents of lava. 
 
 That volcanoes are thus built up along lines of 
 fissure in the earth's crust we have the most convincing 
 proofs. Not only have such fissures been seen in actual 
 course of formation at Vesuvius, Etna, and other active 
 volcanoes, but a study of the volcanoes dissected by 
 denudation affords the most convincing evidence of the 
 same fact. The remarkable linear arrangement of vol- 
 canoes, which is conspicuous to the most superficial 
 observer, is a very striking evidence of the same fact. 
 
s I 
 
 ^ !=> 
 
 J g 
 
 s s 
 
 1 1 
 
 3 S 
 
 I! 
 
 cc 
 
 :? S 
 
 Mfi 
 
 lio! 
 
 H H g 
 
 O S ^ 
 
 J[ *- H 
 
 =f i 
 ill 
 
 J W 
 
SHIFTING OF VENTS ALONG FISSURES. 193 
 
 We have described the action going on at Stromboli 
 as typical of that which occurs at all volcanic vents. 
 Stromboli is, however, one among a group of islands all 
 of which are entirely of volcanic origin. The volcanoes 
 of this group of islands, the ^Eolian or Lipari Islands, 
 are arranged along a series of lines which doubtless 
 mark fissures in the earth's crust. These fissures, as 
 will be seen by the accompanying map (fig. 81), radiate 
 from a centre at which we have proofs of the former 
 
 FIG. 82. THE PUT DE PARIOU IN THE AUVKRGNE, ILLUSTRATING THE 
 SHIFTING OF ERUPTION ALONG A LINK OF FISSURES. 
 
 existence of a volcano of enormous dimensions. It 
 is a very interesting fact, which the studies of Prof. 
 Suess have established, that the earthquakes which 
 have so often desolated Calabria appear to have origi- 
 nated immediately beneath this great centre of volcanic 
 activity. 
 
 When two volcanic cones are thrown up on the same 
 line of fissure, their full development is interfered with, 
 and irregularities in their form and characters are the 
 consequence. In the plan (fig. 82) and the section 
 (fig. 83) an example is given of the results of such a 
 shifting of the centre of eruption along a line of fissure. 
 
194 
 
 VOLCANOES. 
 
 By the second outburst, one-half of the first-formed 
 cone has been removed, and the second-formed overlaps 
 the first. 
 
 FIG. 83. IDEAL SECTION OF THE PUY DE PAUIOU. 
 
 Sometimes a number of scoria- or tuff-cones are 
 thrown up in such close proximity to one another along 
 a line of fissure, that they merge into a long irregular 
 
 I'lG. 84. FlSSURF. FORMED ON THE FLANKS OF ETNA DURING THE 
 ERUPTION OF 1865. 
 
 a. Monte Frumcnto, an old parasitic cone. 6. Line of fissure, c, c, c. New scoria - 
 cones thrown up on line of fissure, d. Lava from sau,e. 
 
 heap on the summit of which a number of distinct 
 craters can be traced. An example of this kind was 
 furnished by the line of scoria-cones formed above the 
 fissure which opened on the flanks of Etna in 1865 
 (see fig. 84). 
 
SHIFTING OF ERUPTIONS ALONG FISSURES. 195 
 
 Even in the case of great composite cones, however, 
 we sometimes find proofs of the centre of eruption 
 having shifted its place along the line of fissure. No 
 
 Scale of English miles. 
 
 FIG. 85. PLAN OF THE ISLAND OF VULCANO, BASED ON THE MAP OF 
 THE ITALIAN GOVERNMENT. 
 
 better example of this kind could possibly be adduced 
 
 ( than that of the Island of Vulcano, with the peninsula 
 
 of Vulcanello, which is joined to it by a narrow isthmus 
 
 (see the map, fig. 81, p. 192). In fig. 85 we have 
 
196 VOLCANOES. 
 
 given an enlarged plan of this island which will make 
 its peculiar structure more intelligible (see also the 
 section given in fig. 77, No. 6, facing p. 178). 
 
 The south-eastern part of the island consists of four 
 crater-rings, one half of each of which has been suc- 
 cessively destroyed, through the shifting of the centre 
 of eruption towards the north-west, along the great 
 line of fissure shown in the general map (fig. 81). 
 The last formed of these four crater-rings is the -one 
 which is now most complete, and culminates in Monte 
 Saraceno (1581 ft.), a in the plan, the highest point in 
 the island. The older crater-rings have been in part 
 removed by the inroads of the waters of the Mediter- 
 ranean on the shores of the island. In the centre of 
 the great crater, 6, which we have just described, rises 
 the present active cone of Vulcano, 1,266 feet high, and 
 having a crater, c, about 600 yards in diameter and 
 more than 500 feet in depth. From this cone, a great 
 stream of obsidian, e, flowed in the year 1775, and a 
 small crater, d, the Fossa Anticha, has been opened in 
 the side of the cone. The continuation of the same 
 line of fissure is indicated by a ruined tuff-cone, /, 
 known as the Faraglione, and the three scoria-cones of 
 Vulcanello, g, A, which have been thrown up so close 
 to one another as to have their lower portions merged 
 in one common mass, as shown in fig. 86. 
 
 Even in volcanoes of the largest dimensions we 
 sometimes find proofs of the centre of eruption having 
 shifted along the line of fissure. Lyell showed that 
 
SYSTEMS OF VOLCANIC FISSURES. 
 
 197 
 
 such a change in the position of the central axis of the 
 volcano had taken place in Etna, and the same pheno- 
 menon is exhibited in the clearest manner by some of 
 the ancient volcanoes of the Inner Hebrides, which 
 have been dissected by the denuding forces. 
 
 a 
 
 FlG. 86. VULCANELLO, WITH ITS THREE CRATERS. 
 
 a. The most recently-formed and perfect crater, b and c. Older craters, the walls 
 of which have been partly removed by denudation, e. Lava-currents proceeding 
 from b. The section exposed in the cliff.at d is represented in fig. 35, p. 116. 
 
 In the case of the Lipari Islands, the fissures along 
 which the volcanic mountains have been thrown up 
 radiate from a common centre, and a similar arrange 
 ment can be traced in many volcanic regions, especially 
 those in which a great central volcano has existed. In 
 other cases, however, as in the Campi Phlegrsei, the 
 volcanic vents appear to be formed along lines which 
 assume a parallel arrangement, and this doubtless 
 marks the relative position of the original fissures pro- 
 
198 VOLCANOES. 
 
 duced in the earth's crust when these volcanoes were 
 formed. In some other cases we find evidences of the 
 existence of a principal fissure from the sides of which 
 smaller cracks originated. These three kinds of ar- 
 rangements of volcano-producing fissures are equally 
 well illustrated when we study those denuded districts, 
 in which, as we have seen, the ground -plans of volcanic 
 structures are revealed to our view. 
 
 There is now good ground for believing that in 
 volcanic vents, at which long-continued eruptive action 
 takes place, the lavas of different chemical composition 
 make their appearance in something like a definite 
 order. It had been remarked by Scrope and other 
 geologists at the beginning of the present century, that 
 in many volcanic areas the acid or trachytic lavas 
 were erupted before the basic or basaltic. 
 
 Von Richthofen, by his studies in Hungary and 
 the volcanic districts of the Rocky Mountains, has been 
 able to enunciate a law governing the natural order of 
 succession of volcanic products ; a,nd although some ex- 
 ception to this law may be mentioned, it is found to 
 hold good for many other districts than those in which 
 it was first determined. 
 
 In a great number of cases it has been found that 
 the first erupted rocks in a volcanic district are those 
 of intermediate composition which are known as ande- 
 sites. These andesites, which are especially charac- 
 terised by the nature of their felspar, sometimes con- 
 tain free quartz and are then known as quartz-andesites 
 
ORDER OF ERUPTION OF VOLCANIC PRODUCTS. 199 
 
 or dacites, from their abundance in Transylvania, the 
 old Roman province of Dacia. 
 
 Von Richthofen suggests that another class of vol- 
 canic rocks, to which he gives the name of ' propylites,' 
 were in every case erupted before the andesites, and 
 in support of his views adduces the fact that in many in- 
 stances propylites are found underlying andesites. But 
 the propylites are, in chemical composition, identical 
 with the andesites, and like them present some varieties 
 in which quartz occurs, and others in which that mine- 
 ral is absent. In their microscopic characters the pro- 
 pylites differ from the andesites and dacites only in the 
 fact that the former are more perfectly crystalline in 
 structure, being indeed in many cases quite undistin- 
 guishable from the diorites or the plutonic representa- 
 tives of the andesites. The propylites also contain 
 liquid cavities, which the andesites and dacites as a 
 rule do not, and the former class of rocks, as Prof. Szabo 
 well points out, are usually much altered by the passage 
 of sulphurous and other vapours, in consequence of 
 which they frequently contain valuable metallic ores. 
 
 The extrusion of these andesitic lavas is sometimes 
 accompanied, and sometimes preceded or followed, by 
 eruptions of trachytic lavas that is, of lavas of inter- 
 mediate composition which have a different kind of 
 felspar from that prevailing in the andesites. 
 
 In the final stages of the eruptive action in most 
 volcanic districts the lavas poured forth belong to the 
 classes of the rhyolitic or acid, and the basaltic or 
 basic lavas. 
 
200 VOLCANOES. 
 
 These facts are admirably illustrated in the case of 
 the volcanic district of the Lipari Islands, to which we 
 have had such frequent occasion to refer. The great 
 central volcano of this district, which now in a ruined 
 condition constitutes a number of small islets (see the 
 map, fig. 81, p. 192), is composed of andesitic lavas. 
 The other great volcanoes thrown up along the three 
 radiating lines of fissure are composed of andesitic and 
 trachytic rocks. But all the more recent ejections of 
 the volcanoes of the district have consisted either of 
 rhyolites, as in Lipari and Vulcano, or of basalts, as in 
 Stromboli. 
 
 Yon Richthofen and the geologists who most strongly 
 maintain the generalisations which he has made con- 
 cerning the order of appearance of volcanic products, go 
 much farther than we have ventured to do, and insist 
 that in all volcanic districts a constant and unvarying 
 succession of different kinds of lavas can be made out. 
 It appears to us, however, that the exceptions to the 
 law, as thus precisely stated, are so numerous as to 
 entirely destroy its value. 
 
 The generalisation that in most volcanic districts the 
 first ejected lavas belong ^ o the intermediate group of 
 the andesites and trachytes, and that subsequently the 
 acid rhyolites and the basic basalts made their appear- 
 ance, is one that appears to admit of no doubt, and is 
 found to hold good in nearly all the volcanic regions 
 of the globe which have been attentively studied. 
 
 The Tertiary volcanic rocks of our own country, those 
 
THEORY OF VOLCANIC MAGMAS. 201 
 
 of North Germany, Hungary, the Euganean Hills, the 
 Lipari Islands, and many other districts in the Old 
 World, together "with the widespread volcanic rocks of 
 the Kocky Mountains in the New World, all seem to 
 conform to this general rule. 
 
 In connection with this subject, it may be well to 
 refer to the ideas on the composition of volcanic rocks 
 which were enunciated by Bunsen, and the theoretic 
 views based on them by Durocher. Bunsen justly 
 pointed out that all volcanic rocks might be regarded 
 as mixtures in varying proportions of two typical kinds 
 of materials, which he named the ' normal trachytic ' 
 and the * normal pyroxenic ' elements respectively. 
 The first of these corresponds very closely in composi- 
 tion with the acid volcanic rocks or rhyolites, and the 
 second with the basic volcanic rocks or basalts. Du- 
 rocher pointed out that if quantities of these different 
 materials existed in admixture, the higher specific 
 gravity of the basic element would cause it gradually to 
 sink to the bottom, while the acid element would rise 
 to the top. Carrying out this idea still further, he pro- 
 pounded the theory that beneath the earth's solid crust 
 there exist two magmas, the upper consisting of light 
 acid materials, the lower of heavy basic ones ; and he 
 supposed that by the varying intensity of the volcanic 
 forces we may have sometimes one or the other magma 
 erupted and sometimes varying mixtures of the two. 
 
 The study of volcanic rocks in recent years has not 
 lent much support to the theoretic views of Durocher 
 10 
 
202 VOLCANOES. 
 
 concerning the existence of two universal magmas be- 
 neath the earth's crust ; and there are not a few facts 
 which seem quite irreconcileable with such a theory. 
 Thus we find evidence that in the adjacent volcanic dis- 
 tricts of Hungary and Bohemia, volcanic action was going 
 on during the whole of the latter part of the Tertiary 
 period. But the products of the contemporaneous vol- 
 canic outbursts in adjacent areas were as different in 
 character as can well be imagined. The volcanic rocks 
 all over Hungary present a strong family likeness ; the 
 first erupted were trachytes, then followed andesites and 
 dacites in great abundance, and lastly rhyolites and 
 basalts containing felspar. But in Bohemia, the lavas 
 poured out from the volcanoes during the same period 
 were firstly phonolites and then basalts containing ne- 
 pheline and leucite. It is scarcely possible to imagine 
 that such very different classes of lavas could have been 
 poured out from vents which were in communication 
 with the same reservoirs of igneous rock, and we are 
 driven to conclude that the Hungarian and Bohemian 
 volcanoes were supplied from different sources. 
 
 But the undoubted fact that in so many volcanic 
 regions the eruption of andesitic and trachytic rocks, 
 which are of intermediate composition, is followed by 
 the appearance of the differentiated products, rhyolite 
 and basalt, which are of acid and basic composition respec- 
 tively, lends not a little support to the view that under 
 each volcanic district a reservoir of more or less com- 
 pletely molten rock exists, and that in these reservoirs 
 
SEPARATION OF LAVAS IN RESERVOIRS. 203 
 
 various changes take place during the long periods of 
 igneous activity. During the earlier period of eruption 
 the heavier and lighter elements of the contents of these 
 subterranean reservoirs appear to be mingled together ; 
 but in the later stages of the volcanic history of the dis- 
 trict, the lighter or acid elements rise to the top, and 
 the heavier or basic sink to the bottom, and we have 
 separate eruptions of rhyolite and basalt. We even 
 find some traces of this action being carried still fur- 
 ther. Among the basalts ejected from the volcanoes 
 of Northern Germany, Bohemia, Styria, Auvergne, and 
 many other regions, we not unfrequently find rounded 
 masses consisting of olivine, enstatite, augite, and other 
 heavier constituents of the rock. These often form the 
 centre of volcanic bombs, and are not improbably por- 
 tions of a dense mass which may have sunk to the 
 bottom of the reservoirs of basaltic materials. 
 
 In consequence of the circumstance that the erup- 
 tion of lavas of intermediate composition usually 
 precedes that of other varieties, we usually find the 
 central and older portions of great volcanoes to be 
 formed of andesites, trachytes, or phonolites, while the 
 outer and newer portions of the mass are made up of acid 
 or basic lavas. This is strikingly exemplified in the 
 great volcanoes of the Auvergne and the Western Isles 
 of Scotland, in all of which we find that great mountain 
 masses have, in the first instance, been built up by extru- 
 sions of lava of the intermediate types, and that through 
 this central core fissures have been opened conveying 
 
204 VOLCANOES. 
 
 basic lavas to the surface. From these fissures great 
 numbers of basaltic lava-streams have issued, greatly 
 increasing the height and bulk of the volcanic cones 
 and deluging the country all around. 
 
 The lavas of intermediate composition the andesites, 
 trachytes, and phonolites possess, as we have already 
 seen, but very imperfect liquidity as they flow from the 
 volcanic vents. Hence we find them either accumulat- 
 ing in great dome-shaped masses above the vent or 
 forming lava-streams which are of great bulk and thick- 
 ness, but do not flow far from the orifices whence they 
 issue. The more fusible basaltic lavas, on the other 
 hand, spread out evenly on issuing from a vent, and 
 sometimes flow to the distance of many miles from it. 
 This difference in the behaviour of the intermediate 
 and basic lavas is admirably illustrated in the volcanic 
 districts of the Auvergne and the Western Isles of 
 Scotland. 
 
 In other cases, like Vesuvius, we find that great 
 volcanic cones of trachytic tuff have been built up, and 
 that these masses of fragmentary trachytic materials 
 have been surrounded and enclosed by the ejection, at 
 a later date, of great outbursts of basaltic lavas. In 
 still other cases, of which Rocca Monfina in Southern 
 Italy constitutes an excellent example, we find that a 
 great crater-ring of trachytic tuffs has been formed in 
 the first instance, and in the midst of this a cone, com- 
 posed of more basic materials, has been thrown up. 
 
 In all these volcanoes we see the tendency towards 
 
EXCEPTIONS TO THE GENERAL LAW. 205 
 
 the eruption of intermediate lavas in the first instance, 
 and of basaltic and acid lavas at a later date. Valuable, 
 however, as are the early generalisations of Scrope, 
 and the more precise law enunciated by Von Kichthofen 
 concerning the ' natural order of succession of volcanic 
 products,' we must not forget that there are to be 
 found a considerable number of exceptions to them. 
 There are some volcanic centres from which only one 
 kind of lava has been emitted, and this may be either 
 acid, basic, or intermediate in composition; and on 
 the other hand, there are districts in which various 
 kinds of lava have been ejected from the same vents 
 within a short period of time, in such a way as to defy 
 every attempt to make out anything like a law as to 
 the order of their appearance. Nevertheless the rules 
 which we have indicated appear to hold good in so 
 great a number of cases that they are well worthy of 
 being remembered, and may serve as a basis on which 
 we may reason concerning the nature of the action 
 going on beneath volcanic vents. 
 
 From the study of the external appearances of vol- 
 canic mountains, combined with investigations of those 
 which have been dissected by denudation, we are able 
 to picture to our minds the series of actions by which 
 the great volcanic mountains of the globe have been 
 slowly and gradually built up. 
 
 In the first instance the eruptions appear to have 
 taken place at several points along a line of fissure, but 
 gradually all of these would become choked up except 
 
206 VOLCANOES. 
 
 one which became the centre of habitual eruption. 
 From this opening, ejections, firstly of lavas of inter- 
 mediate composition, and afterwards of basic materials, 
 would take place, until a volcano of considerable dimen- 
 sions was built up around it. But at last a point would 
 be reached in the piling up of this cone, when the vol- 
 canic forces below would be inadequate to the work 
 of raising the liquid lava through the whole length of 
 the continually upward-growing tube of the volcano. 
 Under these circumstances the expansive force of the 
 imprisoned steam would find it easier to rend asunder 
 the sides of the volcanic cone than to force the liquid 
 material to the summit of the mountain. If these 
 fissures reached the surface explosive action would take 
 place, in consequence of the escape of steam from the 
 glowing mass, and scoria-, tuff-, and lava-cones would 
 be formed above the fissure. In this way, as we have 
 already pointed out, the numerous * parasitic cones' 
 which usually abound on the flanks of the greater vol- 
 canic mountains have been formed. The extrusion of 
 these masses of scoriae and lava on the flanks of the 
 mountain tends, not only to increase the bulk of the 
 mass, but to strengthen and fortify the sides. For by 
 the powerful expansive force at work below, every weak 
 place in the cone is discovered and a fissure produced 
 there ; but by the extrusion of material at this fissure, 
 and still more by the consolidation of the lava in the 
 fissure, the weak place is converted into one of excep- 
 tional strength. 
 
INTKUSIVE MASSES BENEATH VOLCANOES. 207 
 
 As the sides of the cone are thus continually re- 
 paired and strengthened they are rendered more capable 
 of withstanding the heaving forces acting from below, 
 and these forces can then only find vent for themselves 
 by again raising the liquefied lava to the central orifice 
 of the mountain. Many volcanoes, like Etna, exhibit 
 this alternation of eruptive action from the crater at 
 the summit of the mountain, and from fissures opened 
 upon its flanks, the former tending to raise the height 
 of the volcanic pile, the latter to increase its bulk. 
 
 But at last a stage will be reached when the vol- 
 canic forces are no longer able either *to raise the lava 
 up the long column of the central vent on the one 
 hand, or to rend asunder the strongly-built and well- 
 compacted flanks of the mountain on the other. It is 
 probably under these conditions, for the most part, 
 that the lavas find their way between the masses of 
 surrounding strata and force them asunder in the way 
 that we have already described. 
 
 In the case of the more fluid basaltic lavas, as was 
 pointed out so long ago by Macculloch, the liquefied 
 materials may find their way between t}ie strata to 
 enormous distances from the volcanic centre. Such 
 extended flat sheets of igneous rock retain their paral- 
 lelism with the strata among which they are intruded 
 over large areas, and did not probably produce any 
 marked phenomena at the surface. 
 
 But in the case of less fluid lavas, such as those of 
 intermediate or acid composition, for example, the 
 
208 VOLCANOES. 
 
 effect would be far otherwise. Such lavas, not flowing 
 readily from the centre of eruption, would tend to 
 form great bulky lenticular masses between the strata 
 which they forced asunder, and, in so doing, could not 
 fail to upheave and fissure the great mountain-mass 
 above. Vast lenticular masses of trachytic rock, thus 
 evidently forced between strata, have been described 
 by Mr. G. K. Gilbert, as occurring in the Henry 
 Mountains of Southern Utah, and by him have been 
 denominated ' laccolites,' or stone-cisterns. Whether 
 the great basaltic sheets, like those described by 
 Macculloch, and those more bulky lenticular reservoirs 
 of rock of which Mr. Gilbert has given us such an 
 admirable account, were in all cases connected with 
 the surface, may well be a matter for doubt. It is 
 quite possible that, in some cases, liquefied masses of 
 rocky materials in seeking to force their way to the 
 surface only succeeded in thus finding a way for them- 
 selves between the strata, and their energy was ex- 
 pended before the surface was reached and explosive 
 action took place. But it is an undoubted fact that 
 beneath many of the old volcanoes, of which the in- 
 ternal structure is now revealed to us by the action of 
 denuding forces, great intrusive sheets and laccolites 
 abound ; and we cannot doubt that beneath volcanoes 
 now in a state of eruption, or in those which have but 
 recently become extinct, similar structures must be in 
 course of formation. 
 
 That great upheaving forces have operated on vol- 
 
EFFECTS OF INTRUSION BENEATH CONES. 209 
 
 canoes, subsequently to the accumulation of their 
 materials, we have sufficient evidence in the Val del 
 Bove of Etna, the Caldera of Palma, the Ourral of 
 Madeira, &c. In all of these cases we find a radial 
 fissure ('barranco') leading into a great crateral 
 hollow ; and these radial fissures are of such width and 
 depth that their origin can only be referred to a dis- 
 ruptive force like that which would be exercised by 
 the intrusion of masses of more or less imperfectly 
 fluid material between the subjacent strata. These 
 facts, of course, lend no countenance to the views 
 formerly held by many geologists, both in Germany and 
 France, that the materials of which volcanoes are built 
 up were deposited in an approximately horizontal po- 
 sition, and were subsequently blown up like a gigantic 
 bubble. In Etna, Palma, and Madeira we find abun- 
 dant proofs that the mass existed as a great volcanic 
 cone before the production of the fissures (barrancos), 
 which we have referred to the force exercised during 
 the intrusion of great igneous masses beneath them. 
 
 But besides the horizontally-disposed intrusive 
 sheets and laccolites, great, radiating, vertical fissures 
 are produced by the heaving forces acting beneath 
 those volcanic centres which have been closed up 
 and ' cicatrised ' by the exudation from them of sub- 
 terranean materials. These vertical intrusions, which 
 we call dykes, like the horizontal ones, differ in cha- 
 racter, according to the nature of the materials of 
 which they are composed. Dykes of acid and inter- 
 
210 VOLCANOES. 
 
 mediate lava are usually of considerable width, and 
 do not extend to great distances from the centres of 
 eruption. Dykes composed of the more-liquid, basic 
 lavas, on the other hand, may extend to the distance 
 of hundreds of miles from the central vent. The way 
 in which comparatively narrow, basaltic dykes are found 
 running in approximately straight lines for such 
 enormous distances is a very striking fact, and bears 
 the strongest evidence to the heaving and expanding 
 forces at work at volcanic centres, during and subse- 
 quently to the extrusion of the igneous products at 
 the surface. 
 
 These basaltic dykes occur in such prodigious 
 numbers around some volcanic vents, that the whole 
 of the stratified rocks in the immediate vicinity are 
 broken up by a complete network of them, crossing 
 and interlacing in the most complicated fashion. 
 Farther away from the vents, similar dykes are found 
 in smaller numbers, evidently radiating from the same 
 centre, and sometimes extending to a distance of 
 more than a hundred miles from it. Nowhere can we 
 find more beautiful illustrations of such dykes than 
 in the Western Isles of Scotland. When composed of 
 materials which do not so easily undergo decom- 
 position as the surrounding rocks, they stand up like 
 vast walls ; but when, on the other hand, they are 
 more readily acted on by atmospheric moisture than are 
 the rocks which enclose them, they give rise to deep 
 trenches with vertical sides, which render the country 
 almost impassable. 
 
STKUCTURE OF INTKUSIVE MASSES. 211 
 
 The lava consolidating in these horizontal intrusions 
 (sheets and laccolites), and the vertical intrusions 
 (dykes), is usually more crystalline in structure than 
 the similar materials poured out at the surface. In 
 the same dyke or sheet, when it is of great width, 
 we often find every variation from a glassy material 
 formed by the rapid cooling of the mass where it is in 
 contact with other rocks, to the perfectly crystalline 
 or granitic varieties which form the centre of the in- 
 trusion. It is in these dykes and other intrusions 
 that we find the most convincing evidence of the truth 
 of the conclusions, which we have enunciated in a 
 former chapter, concerning the dependence of the 
 structure of an igneous rock upon the conditions 
 under which it has consolidated. One material is 
 found, under varying conditions, assuming the cha- 
 racters of obsidian, rhyolite, quartz-felsite, or granite ; 
 another, under the same set of conditions, taking the 
 form of tachylyte, basalt, dolerite, and gabbro. 
 
 That these great intrusive masses, sheets and dykes, 
 in their passage between the sedimentary rocks some- 
 times find places where the overlying strata are of 
 such thinness or incoherence that the liquified rocks are 
 able to force a way for themselves to the surface, we 
 have the clearest proof. In some dykes we find the 
 rock in their upper portions losing its compact charac- 
 ter and becoming open and scoriaceous, showing that 
 the pressure had been so far diminished as to allow of 
 the imprisoned water flashing into steam. 
 
212 VOLCANOES. 
 
 All round great volcanoes which have become ex- 
 tinct we frequently find series of small volcanic cones, 
 which have evidently been thrown up along the lines 
 where the great lava-filled fissures, which we have been 
 describing, have reached the surface and given rise to 
 explosive action there. The linear arrangement of 
 these small cones, which are thrown up in the plains 
 surrounding vast volcanic mountains that have become 
 extinct, is very striking. The numerous ' puys ' of the 
 Auvergne and adjoining volcanic regions of Central 
 France are for the most part small scoria- and lava- 
 cones which were thrown up along great lines of fissure 
 radiating from the immense, central, volcanic mountains 
 of the district, after they had become extinct. These 
 scoria-cones and the small lava-streams which flow from 
 them, as was so well shown by Mr. Scrope, mark the 
 latest efforts of the volcanic forces beneath the district 
 before they finally sank into complete extinction. In 
 the Western Isles of Scotland, as I have elsewhere 
 shown, we can study the formation of these later-formed 
 cones in the plains around extinct volcanic mountains, 
 with the additional advantage of having revealed to 
 us, by the action of the denuding forces, their connec- 
 tion with the great radiating fissures. 
 
 It has been shown that the several stages in the 
 decline of each volcanic outburst is marked by the 
 appearance at the vent of certain acid gases. In the 
 same way, after the ejection of solid materials from a 
 volcanic vent has come to a.n end, certain gaseous sub- 
 
OKDEE OF EMISSION OF VOLCANIC GASES. 213 
 
 stances continue to be evolved ; and as the temperature 
 at the vents declines, the nature of the volatile sub- 
 stances emitted from them undergoes a regular series 
 of changes. 
 
 M. Fouque, by a careful series of analyses of the 
 gases which he collected at different gaseous vents, or 
 fumaroles as they are called, in the crater of Vulcano, 
 has been able to define the general relations which 
 appear to exist between the temperature at a volcanic 
 orifice and the volatile substances which issue from it. 
 He found that in fumaroles, in which the temperature 
 exceeded 360 centigrade, and in which in consequence 
 strips of zinc were fused by the stream of issuing gas, 
 the analysis of the products showed sulphurous acid 
 and hydrochloric acid to be present in large quantities, 
 and sulphuretted hydrogen and carbonic acid in much 
 smaller proportions. Around these excessively heated 
 fumaroles, the lips of which often appear at night to 
 be red-hot, considerable deposits of sulphide of arsenic, 
 chloride of iron, chloride of ammonium, boracic acid, 
 and sulphur were taking place. 
 
 It was found, however, that as the temperature of 
 the vent declined, the emission of the sulphurous acid 
 and hydrochloric acid diminished, and the quantity of 
 sulphuretted hydrogen and carbonic acid mingled with 
 them was proportionately increased. 
 
 In the same way it appears to be a universal rule 
 that when a volcanic vent sinks into a condition of 
 temporary quiescence or complete extinction the power- 
 
214 VOLCANOES. 
 
 fully acid gases, hydrochloric acid and sulphurous acid, 
 make their appearance in the first instance, and at a 
 later stage these are gradually replaced by sulphuretted 
 hydrogen and carbonic acid. 
 
 Of these facts we find a very beautiful illustration 
 in the Campi Phlegrsei near Naples. With the ex- 
 ception of Monte Nuovo, the volcano which has most 
 recently been in a state of activity in that district is 
 the Solfatara. From certain apertures in the floor of 
 the crater of the Solfatara there issue continually watery 
 vapours, sulphurous acid, sulphuretted hydrogen, hydro- 
 chloric acid, and chloride of ammonium. The action of 
 these substances upon one another, and upon the vol- 
 canic rocks through which they pass, gives rise to the 
 formation of certain chemical products which, from a 
 very early period, have been collected on account of 
 their commercial value. The action of these acid gases 
 upon the surrounding rocks is very marked ; efflores- 
 cent deposits of various sulphates and chlorides take 
 place in all the crevices and vesicles of the rock ; sul- 
 phur and sulphide of arsenic are also formed in consider- 
 able quantities ; and the trachytic tuffs, deprived of 
 their iron-oxide, alkaline earths and alkalies, which 
 are converted into soluble sulphates and chlorides, are 
 reduced to a white, powdery, siliceous mass. Many vol- 
 canoes, which have sunk into a state of quiescence or 
 extinction like the Solfatara of Naples, exhibit the same 
 tendency to give off great quantities of the powerfully- 
 acid gases which act upon the surrounding rocks, and 
 
SOLFATAKA-STAG-E OF VOLCANOES. 215 
 
 deprive them of their colour and consistency. Such 
 volcanoes are said by geologists to have sunk into the 
 ' solfatara stage.' 
 
 At the Lake of Agnano and some other points in the 
 Campi Phlegrsei, however, we find fissures from which 
 the less-powerfully acid gases, sulphuretted hydrogen 
 and carbonic acid, issue. These gases as they are 
 poured forth from the vents are found to be little, if at 
 all, above the temperature of the atmosphere. Sulphu- 
 retted hydrogen is an inflammable gas, and in the so- 
 called salses and mud-volcanoes, at which it is ejected 
 in considerable quantities, it not unfrequently takes fire 
 and burns with a conspicuous flame. Carbonic acid on 
 account of its great density tends to accumulate in 
 volcanic fissures and craters rather than to mingle with 
 the surrounding atmosphere. At the so-called Grotto 
 del Cane, beside the Lago Agnano, it is the custom to 
 show the presence of this heavy and poisonous gas by 
 thrusting a dog into it, the poor animal being revived, 
 before life is quite extinct, by pouring cold water over 
 it. At the Biidos Hegy or stinking hill ' of Transyl- 
 vania, carbonic acid and sulphuretted hydrogen are 
 emitted in considerable quantities, and it is possible 
 to take a bath of the heavy gas, the head being kept 
 carefully above the constant level of the exhalations. 
 
 Although the stories of the ancient Avernian lake, 
 across which no bird could fly without suffocation, and 
 of the Guevo Upas, or Poison Valley of Java, which it 
 has been said no living being can cross, may not im- 
 
216 VOLCANOES. 
 
 probably be exaggerations of the actual facts, yet there 
 is a basis of truth in them in the existence of old vol- 
 canic fissures and craters which evolve the poisonous 
 sulphuretted hydrogen and carbonic acid gases. 
 
 Besides the gases which we have already named, 
 and which are the most common at and characteristic 
 of volcanic vents, there are some others which are not 
 unfrequently emitted. First among these we must 
 mention boracic acid, which, though not a remarkably 
 volatile substance, is easily carried along in a fine state 
 of division in a current of steam. At Monte Cerboli 
 and Monte Eotondo in Tuscany, great quantities of 
 steam jets accompanied by sulphuretted hydrogen and 
 boracic acid issue from the rocks, and these jets being 
 directed into artificial basins of water, the boracic acid 
 is condensed and is recovered by evaporation. We 
 have already noticed that boracic acid is evolved with 
 the gases at Vulcano and other craters ; and the part 
 which this substance plays in volcanic districts is shown 
 by the fact that many of the rocks, filling old subter- 
 ranean volcanic reservoirs, are found to be greatly 
 altered and to have new minerals developed in their 
 midst through the action upon them of boracic acid. 
 
 Ammonia and various compounds of carbon, nitro- 
 gen, and hydrogen are among the gases evolved from 
 volcanic vents. In some cases these gases may be pro- 
 duced by the destructive distillation of organic mate- 
 rials in the sedimentary rocks through which volcanic 
 outbursts take place. But it is far from impossible 
 
GEYSERS AND HOT-SPRINGS. 217 
 
 that under the conditions of temperature and pressure 
 which exist at the volcanic foci, direct chemical union 
 may take place between substances, which at the sur- 
 face appear to be perfectly inert in each other's pre- 
 sence. 
 
 When the temperature at volcanic fissures is no 
 longer sufficiently high to cause water to issue in the 
 condition of vapour or steam, as is the case at the 
 'stufas' which we have described, it comes forth in 
 the liquid state. Water so issuing from old volcanic 
 fissures may vary in its temperature, from the boiling 
 point downwards. 
 
 When the water issues at a temperature little re- 
 moved from the boiling point, it is apt to give rise 
 to intermittent springs or geysers, the eruptions of 
 which exhibit a remarkable analogy with those of 
 ordinary volcanoes. Geysers may indeed be described 
 as volcanoes in which heated water, instead of molten 
 rock, is forced out from the vent by the escaping steam. 
 They occur in great abundance in districts in which the 
 subterranean action is becoming dormant or extinct, 
 such as Iceland, the North Island of New Zealand, 
 and the district of the National Park in the Eocky 
 Mountains. 
 
 Many attempts have been made to explain the 
 exact mechanism by which the intermittent action of 
 geysers is produced, but it is not at all probable that 
 any one such explanation will cover all the varied phe- 
 nomena exhibited by them. Like volcanic outbursts, 
 
218 VOLCANOES. 
 
 geyser eruptions doubtless originate in the escape of 
 bubbles of steam through a liquid mass, and this libera- 
 tion of steam follows any relief of pressure. In districts 
 where vast masses of lava are slowly cooling down from 
 a state of incandescence, and surface waters are finding 
 their way downwards while subterranean waters are 
 finding their way upwards, there can be no lack of the 
 necessary conditions for such outbursts. Sometimes 
 the eruptions of geysers take place at short and regular 
 intervals, at other times they occur at wide and irregular 
 intervals of time. In some cases the outbursts take 
 place spontaneously, and at others the action can be 
 hastened by choking up the vent with stones or earth. 
 Other hot springs, like the Strudel of Carlsbad, rise 
 above the surface in a constant jet, while most of them 
 issue quietly and flow like ordinary springs. 
 
 Although the violent and paroxysmal outbursts of 
 volcanic mountains arrest the attention, and power- 
 fully impress us with a sense of the volcanic activity 
 going on beneath the earth's surface, yet it may well 
 be doubted whether the quantity of heat, which the 
 earth gets rid of by their means, at all approaches in 
 amount that which is quietly dissipated by means of 
 the numerous stufas,' gaseous exhalations, and thermal 
 springs which occur in such abundance all over its 
 surface. For while the former are intermittent in their 
 action, and powerful outbursts are interrupted by long 
 periods of rest, the action of the latter, though feeble, 
 is usually continuous. 
 
EFFECTS OF HOT-SPKINGS. 219 
 
 Most people may regard the hot spring of Bath as 
 a very slight manifestation of volcanic activity. This 
 spring issues at a constant temperature of 49 0., or 
 120 Fahr. As, however, no less than 180,000 gallons 
 of water issue daily from this source, we may well un- 
 derstand how great is the amount of heat of which the 
 earth's crust is relieved by its agency. It may in- 
 deed be doubted whether its action in this way is not 
 at least equal to that of a considerable volcano which, 
 though so much more violent, is intermittent in its 
 action. 
 
 Nor are thermal springs by any means ineffective 
 agents in bringing materials from the interior of the 
 earth's crust and depositing it at the surface. The 
 Bath spring contains various saline substances, prin- 
 cipally sulphates and chlorides, in solution in its 
 waters. These are quietly carried by rivers to the sea, 
 and are lost to our view. The spring has certainly 
 maintained its present condition since the time of the 
 Eomans, and I find that if the solid materials brought 
 from the interior of the earth during the last 2,000 
 years had been collected, they would form a solid cone 
 equal in height to Monte Nuovo. Yet we usually re- 
 gard the Campi Phlegrsei as a powerfully-active volcanic 
 district, and the subterranean action in our own country 
 as quite unworthy of notice. 
 
 When we remember the fact that on the continent 
 of Europe the hot and saline springs may be numbered 
 by thousands, and that they especially abound in districts 
 
220 VOLCANOES. 
 
 like Hungary, the Auvergne, the Ehine provinces, and 
 Central Italy, where volcanic action has recently become 
 extinct, we shall be able to form some slight idea of the 
 work performed by these agents, not only in relieving the 
 earth's crust of its superfluous heat, but in transporting 
 materials in a state of solution from the interior of that 
 crust and depositing them at the surface. The vast 
 deposits of siliceous sinter and of travertine also bear 
 witness to the effects produced by hot and mineral 
 springs. 
 
 Nor is the work of these springs confined to the 
 surface. Mr. John Arthur Phillips has shown that 
 metallic gold and the sulphide of quicksilver (cinna- 
 bar) have been deposited with the silica and other 
 minerals formed on the sides of a fissure from which 
 hot springs issue at the surface. There cannot be any 
 doubt that the metallic veins or lodes, which are the 
 repositories of most of the metals employed in the arts, 
 have been formed in cracks connected with great vol- 
 canic foci, the transfer of the various sulphides, oxides, 
 and salts which fill the vein having been effected either 
 by solution, sublimation, or the action of powerful cur- 
 rents of steam. 
 
 As the igneous activity of the district declines, the 
 temperature of the issuing gases and waters diminishes 
 with it, until at last the volcanic forces appear to wholly 
 abandon that region and to be transferred to another. 
 
 Yet even after all or nearly all indications of the 
 volcanic agencies cease to make themselves visible at 
 
NATUEE OF VOLCANIC CYCLES. 221 
 
 the surface, occasional tremblings of the earth's crust 
 show that perfect equilibrium has not been restored 
 below, but that movements are taking place which 
 result in shocks that are transmitted through the 
 overlying and surrounding rock-masses as earthquake 
 vibrations. 
 
 Such is the cycle of changes which appears to take 
 place at each district of the earth's surface, as it suc- 
 cessively becomes the scene of volcanic activity. 
 
 The invasion of any particular area of the earth's 
 surface by the volcanic forces appears to be heralded 
 by subterranean shocks causing earthquake vibrations. 
 Presently the origination of fissures is indicated by the 
 rise of saline and thermal springs, and the issuing of 
 carbonic acid and other gases at the surface. As the 
 subterranean activity becomes more pronounced, the 
 temperature of the springs and emitted gases is found 
 to increase, and at last a visible rent is formed at the 
 surface, exposing the incandescent materials below. 
 
 From this open fissure which has thus been formed, 
 the gas and vapours imprisoned in the incandescent 
 rock-materials escape with such violence as to disperse 
 the latter in scoriae and dust, or to cause them to well out 
 in great streams as lava-flows. Usually the action be- 
 comes concentrated at one or several points at which 
 the ejected materials accumulate to form volcanic 
 cones. 
 
 Sometimes the volcanic activity dies away entirely 
 after these cones are thrown up along the line of fissure, 
 
222 VOLCANOES. 
 
 but at others some such centre becomes for a longer or 
 shorter time the habitual vent for the volcanic forces 
 in the district, and by repeated ejections of lavas and 
 fragmentary materials at longer or shorter intervals 
 the cone increases both in height and bulk. 
 
 When the height of the cone has grown to a certain 
 extent, it becomes more easy for the volcanic energies 
 below to rend the sides of the cone than to raise the 
 molten materials to its summit. In this way lateral 
 or parasitic cones are thrown up on the flanks of the 
 volcanic mountain, the mass being alternately elevated 
 and strengthened by the ejections from the summit 
 and sides respectively. 
 
 When the volcanic energies no longer suffice to 
 raise the fluid materials to the summit, nor to rend the 
 sides of the volcano, fissures with small cones may be 
 formed in the plains around the great central volcano. 
 At last, however, this energy diminishes so far that 
 rock materials can no longer be forced to the surface, 
 the fissures become sealed up by consolidating lava, and 
 the volcanic cones fall into a condition of extinction 
 and decay. 
 
 The existence of heated materials at no great depth 
 from the surface is indicated by the outburst of gases and 
 vapours, the formation of geysers, mud-volcanoes, and 
 ordinary thermal springs. But as the underlying rocks 
 cool down, the issuing jets of gas and vapour lose their 
 high temperature and diminish in quantity, the geysers 
 and mud-volcanoes become extinct, and the thermal 
 
DUKATION OF VOLCANIC CYCLES. 223 
 
 springs lose their peculiar character or disappear, and 
 thus all manifestations of the igneous energies in the 
 district gradually die away. 
 
 Such a cycle of changes probably requires many 
 hundreds of thousands, or even many millions, of years 
 for its accomplishment ; but by the study of volcanoes 
 in every stage of their growth and decline we are 
 able to reconstruct even the minutest details of their 
 history. 
 
224 VOLCANOES. 
 
 CHAPTER VIII. 
 
 THE DISTRIBUTION OF VOLCANOES UPON THE SURFACE 
 OF THE GLOBE. 
 
 If is not by any means an easy task to frame an 
 estimate of the number of volcanoes in the world. 
 Volcanoes, as we have seen, vary greatly in their 
 dimensions from vast mountain masses, rising to a 
 height of nearly 25,000 feet above the sea-level, to 
 mere molehills ; the smaller ones being in many cases 
 subsidiary to larger, and constituting either parasitic 
 cones on their flanks, or ' puys ' around their bases. 
 Volcanoes likewise exhibit every possible stage of de- 
 velopment and decay: while some are in a state of 
 chronic active eruption, others are reduced to the con- 
 dition of solfataras, and others again have fallen into 
 a more or less complete state of ruin through the 
 action of denuding forces. 
 
 Even if we confine our attention to the larger 
 volcanoes, which merit the name of ' mountains,' and 
 such of these as we have reason to beJieve to be in a 
 still active condition, our difficulties will be diminished, 
 but not by any means removed. Volcanoes, as we have 
 
NUMBER OF ACTIVE VOLCANOES. 225 
 
 seen, may sink into a dormant condition that may 
 endure for hundreds or even thousands of years, and 
 then burst forth into a state of renewed activity; and 
 it is quite impossible, in many cases, to distinguish 
 between the conditions of dormancy and extinction. 
 Concerning certain small areas in Southern Europe, 
 Western Asia, and Northern Africa, historical records, 
 more or less reliable, extend back over periods of 
 several thousands of years ; but with regard to the 
 greater part of the rest of the world we have no in- 
 formation beyond a few hundred years, and there are 
 considerable areas which have been known only for far 
 shorter periods, while some are as yet quite unexplored. 
 In districts almost wholly uninhabited, or roamed over 
 by nomadic tribes, legend and tradition constitute our 
 only guides and very unsafe ones they are in the 
 attempt to determine what volcanoes have recently 
 been in a condition of activity. 
 
 We shall, however, probably be within the limits 
 of truth in stating that the number of great habitual 
 volcanic vents upon the globe, which we have reason 
 to believe are still in an active condition, is somewhere 
 between 300 and 350. Most of these active volcanic 
 vents are marked by more or less considerable moun- 
 tains, composed of the materials ejected from them. 
 If we include the mountains which exhibit the ex- 
 ternal conical form, the crateral hollows, and other 
 features of volcanoes, but concerning the activity of 
 which we have no record or tradition, the number will 
 11 
 
226 VOLCANOES. 
 
 fall little, if anything, short of 1,000. The mountains 
 composed of volcanic materials, but which have lost 
 through denudation the external form of volcanoes, are 
 still more numerous. The smaller temporary openings 
 which are usually subordinate to the habitual vents, 
 that have been active during the. periods covered by 
 history and tradition, must be numbered by thousands 
 and tens of thousands. The still feebler manifesta- 
 tions of the volcanic forces such as are exhibited in 
 ' stufas,' or steam-jets, geysers, or intermittent hot 
 springs, thermal and mineral waters, fumaroles, emit- 
 ting various gases, salses or spouting saline and muddy 
 springs, and mud volcanoes may be reckoned by 
 millions. It is not improbable that these less powerful 
 manifestations of the volcanic forces, to a great extent 
 make up in number what they want in individual 
 energy ; and the relief which they afford to the im- 
 prisoned activities within the earth's crust may be 
 scarcely less than that which results from the occa- 
 sional outbursts at the 300 or 350 great habitual 
 volcanic vents. 
 
 In taking a general survey of the volcanic pheno- 
 mena of the globe, no fact comes out more strikingly 
 than that of the very unequal distribution, in different 
 districts, both of the great habitual volcanic vents, and 
 of the minor exhibitions of subterranean energy. 
 
 Thus, on the whole of the continent of Europe, 
 there is but one habitual volcanic vent that of 
 Vesuvius and this is situated upon the shores of the 
 
VOLCANOES OF THE CONTINENTS. 227 
 
 Mediterranean. In the islands of the Mediterranean, 
 however, there are no less than six volcanoes ; namely, 
 Strom boli and Vukano, in the Lipari Islands ; Etna, in 
 Sicily; Graham's Isle, a submarine volcano, off the 
 Sicilian coast ; and Santorin and Nisyros, in the ^Egean 
 Sea. 
 
 The African continent is at present known to con- 
 tain about ten active volcanoes four on the west 
 coast, and six on the east coast; about ten other 
 active volcanoes occur on islands close to the African 
 coasts. In Asia, twenty-four active volcanoes are 
 known, but no less than twelve of these are situated 
 in the peninsula of Kamtschatka. No volcanoes are 
 known to exist in the Australian continent. 
 
 The American continent contains a greater number 
 of volcanoes than the divisions of the Old World. 
 There are twenty in North America, twenty-five in 
 Central America, and thirty-seven in South America. 
 
 Thus, taken altogether, there are about one hundred 
 and seventeen volcanoes situated on the great conti 
 nental lands of the globe, while nearly twice as many 
 occur upon the islands scattered over the various 
 oceans. 
 
 Upon examining further into the distribution of 
 the continental volcanoes, another very interesting 
 fact presents itself. The volcanoes are in almost 
 every case situated either close to the coasts of the 
 continent, or at no great distance from them. There 
 are, indeed, only two exceptions to this rule. In the 
 
228 VOLCANOES. 
 
 great and almost wholly unexplored table-land lying 
 between Siberia and Tibet four volcanoes are said to 
 exist, 'and in the Chinese province of Mantchouria 
 several others. More reliable information is, however, 
 needed concerning these volcanoes, situated, unlike all 
 others, at a great distance from the sea. 
 
 It is a remarkable circumstance that all the oceanic 
 islands which are not coral-reefs are composed of 
 volcanic rocks ; and many of these oceanic islands, as 
 well as others lying near the shores of the continents, 
 contain active volcanoes. 
 
 Through the midst of the Atlantic Ocean runs a 
 ridge, which, by the soundings of the various ex- 
 ploring vessels sent out in recent years, has been 
 shown to divide the ocean longitudinally into two 
 basins. Upon this great ridge, and the spurs pro- 
 ceeding from it, rise numerous mountainous masses, 
 which constitute the well-known Atlantic islands and 
 groups of islands. All of these are of volcanic origin, 
 and among them are numerous active volcanoes. The 
 Island of Jan Mayen contains an active volcano, and 
 Iceland contains thirteen, and not improbably more ; 
 the Azores have six active volcanoes, the Canaries 
 three; while about eight volcanoes lie off the west 
 coast of Africa. In the West Indies there are six 
 active volcanoes ; and three submarine volcanoes have 
 been recorded within the limits of the Atlantic Ocean. 
 Altogether, no less than forty active volcanoes are 
 situated upon the great submarine ridges which tra- 
 verse the Atlantic longitudinally. 
 
VOLCANOES ON THE OCEANIC ISLANDS. 229 
 
 But along the same line the number of extinct 
 volcanoes is far greater, and there are not wanting 
 proofs that the volcanoes which are still active are 
 approaching the condition of extinction. At a some- 
 what earlier period of the earth's history the whole 
 line of the present Atlantic Ocean was in all proba- 
 bility traversed by a chain of volcanoes on the very 
 grandest scale ; but submergence has taken place, and 
 only a few portions of this great mountain range now 
 rise above the sea-level, forming the isolated islands 
 and island-groups of the Atlantic. Here and there 
 among these a still active volcano exists. 
 
 But if the great medial chain of the Atlantic pre- 
 sents us with an example of a chain of volcanic moun- 
 tains verging on extinction, we have in the line of 
 islands separating the Pacific and Indian Oceans an 
 example of a similar range of volcanic vents which are 
 in a condition of the greatest activity. In the penin- 
 sula of Kamtschatka there are twelve active volcanoes, 
 in the Aleutian Islands thirty-one, and in the peninsula 
 of Alaska three. The chain of the Kuriles contains at 
 least ten active volcanoes; the Japanese Islands and 
 the islands lying to the south of Japan twenty-five. 
 The great group of islands lying to the south-east of 
 the Asiatic continent is at the present time the grandest 
 focus of volcanic activity upon the globe. No less 
 than fifty active volcanoes occur here. Farther south, 
 the same chain is probably continued by the four 
 active volcanoes of New Guinea, one or more sub- 
 
230 VOLCANOES. 
 
 marine volcanoes, and several vents in New Britain, 
 the Solomon Isles, and the New Hebrides, the three 
 active volcanoes of New Zealand, and possibly by 
 Mount Erebus and Mount Terror in the Antarctic 
 region. Altogether, no less than 150 active volcanoes 
 exist in the chain of islands which stretch from 
 Behring s Straits down to the Antarctic circle ; and if 
 we include the volcanoes on Indian and Pacific islands 
 which appear to be situated on lines branching from 
 this particular band, we shall not be wrong in the 
 assertion that this great system of volcanic mountains 
 includes at least one half of the habitually active vents 
 of the globe. 
 
 A third series of volcanoes starts from near the 
 last in the neighbourhood of Behring's Straits, and 
 stretches along the whole western coast of the Ameri- 
 can continent. In this great range there are about 
 eighty active volcanoes. 
 
 In considering the facts connected with the distribu- 
 tion of volcanoes upon the globe, the one which, by its 
 striking character, seems to demand our attention in 
 the first instance is that of the remarkable linear 
 arrangement of volcanic vents. We have already seen 
 that small scoria-cones are often thrown up on the 
 flanks, or at the base, of a great volcanic mountain, 
 along lines which are manifestly lines of fissure. In 
 the eruption of Etna, in 1865, and again in that of 
 1874, Professor Silvestri, of Catania, witnessed the 
 actual opening of great fissures on the north-east and 
 
LINEAR ARRANGEMENT OF VOLCANOES. 231 
 
 north sides of the mountain ; and along the bottom of 
 these cracks the glowing lava was clearly visible (fig. 
 84, page 194). In the course of a few days, there were 
 thrown up a number of small scoria-cones along these 
 lines of fissure those formed on the fissure of 1865 
 being seven in number, and those on the fissure of 1874 
 being no less than thirty-six in number. Precisely 
 similar phenomena were witnessed upon the slopes of 
 Vesuvius, in 1760, when a fissure opened on the south 
 side of the mountain, and fifteen scoria-cones, which 
 are still visible, were thrown up along it. 
 
 We have already considered the evidence pointing 
 to the conclusion that systems of volcanoes, like that 
 of the Lipari Islands, are similarly ranged along lines of 
 fissures, and there is equally good ground for believing 
 that the great linear bands of volcanoes, which, as we 
 have seen, stretch for thousands of miles, have had 
 their positions determined by great lines of fissure in 
 the earth's crust. While, however, the smaller fissures, 
 upon which rows of scoria-cones are thrown up, seem 
 to have been in many cases opened by a single effort 
 of the volcanic forces, the enormous fissures, which 
 traverse so large a portion of the surface of the globe, 
 are doubtless the result of numerous manifestations of 
 energy extending over vast periods of time. 
 
 The greatest of these bands along which the volcanic 
 forces are so powerfully exhibited at the present day, is 
 the one which stretches from near the Arctic circle 
 at Behring's Straits to the Antarctic circle at South 
 
232 VOLCANOES. 
 
 Victoria. The line followed by this volcanic band, which, 
 as we have seen, includes more than one half of the 
 active volcanoes of the globe, is a very sinuous one, 
 and it gives off numerous offshoots upon either side of 
 it. The great focus of this intense volcanic action may 
 be regarded as lying in the district between the islands 
 of Borneo and New Guinea. From this centre there 
 radiate a number of great lines, along which the 
 volcanic forces are exhibited in the most powerful 
 manner. The first of these extends northwards through 
 the Philippine Isles, Japan, the Kurile Islands, and 
 Kamtschatka, giving off a branch to the east, which 
 passes through the Aleutian Islands and the peninsula 
 of Alaska. This band, along which the volcanic forces 
 are very powerfully active, is continued towards the 
 south-east in the New Britain, the Solomon Islands, 
 Santa Cruz, the New Hebrides, New Zealand, and South 
 Victoria. East and west from the great central focus 
 there proceed two principal branches. The former of 
 these extends through the Navigator Islands and 
 Friendly Islands as far as Elizabeth Islands. The latter 
 passes through Java, and then turns north-westward 
 through Sumatra, the Nicobar Islands, the Andaman 
 Islands up to the coast of Burmah. 
 
 The great band which we have been describing 
 exhibits the most striking examples of volcanic activity 
 to be found upon the globe. Besides the 150 or more 
 volcanoes which are known to have been in a state of 
 activity during the historical period, there are several 
 
GEE AT VOLCANIC BANDS OF THE GLOBE. 233 
 
 hundred very perfect volcanic cones, many of which 
 appear to have but recently become extinct, if indeed, 
 they are not simply in a dormant condition. For long 
 distances these chains of volcanic cones are almost con- 
 tinuous, and the only very considerable breaks in the 
 series are those between New Zealand and the New 
 Hebrides on the one hand, and between the former 
 islands and South Victoria on the other. 
 
 Much less continuous, but nevertheless very im- 
 portant, is the great band of volcanoes which extends 
 along the western side of the great American continent, 
 and contains, with its branches, nearly a hundred active 
 volcanoes. On the north this great band is almost 
 united with the one we have already described by 
 the chain of the Aleutian and Alaska volcanoes. In 
 British Columbia about the parallel of 60 N. there 
 exist a number of volcanic mountains, one of which, 
 Mount St. Elias, is believed to be 18,000 feet in height, 
 and several of these have certainly been seen in a state 
 of eruption. Farther south in the part of the United 
 States, territories drained by the Columbia Kiver, a num- 
 ber of grand volcanic mountains exist, some of which 
 are probably still active, for geysers and other manifes- 
 tations of volcanic activity abound. From the southern 
 extremity of the peninsula of California an almost 
 continuous chain of volcanoes stretches through Mexico 
 and Guatemala, and from this part of the volcanic 
 band a branch is given off which passes through the 
 West Indies, and forms a connection with the great 
 
234 VOLCANOES. 
 
 volcanic band of the Atlantic Ocean. In South America 
 the line is continued by the active volcanoes of Ecuador, 
 Bolivia and Chili, but at many intermediate points in 
 the chain of the Andes extinct volcanoes occur, which to 
 a great extent fill up the gaps in the series. A small 
 offshoot to the westward passes through the Galapagos 
 Islands. The great band of volcanoes which stretches 
 through the American continent is second only in im- 
 portance, and in the activity of its vents, to the band 
 which divides the Pacific from the Indian Ocean. 
 
 The third volcanic band of the globe is that which 
 traverses the Atlantic Ocean from north to south. 
 This series of volcanic mountains is much more broken 
 and interrupted than the other two, and a greater 
 proportion of its vents are extinct. This chain, as we 
 shall show in a future chapter, attained its condition of 
 maximum activity during the distant period of the 
 Miocene, and' now appears to be passing into a state of 
 gradual extinction. Beginning in the north with the 
 volcanic rocks of Greenland and Bear Island, we pass 
 southwards, by way of Jan May en, Iceland, and the 
 Faroe Islands, to the Hebrides and the north of Ireland. 
 Thence by way of the Azores, the Canaries and the 
 Cape de Verde Islands, with some active vents, we 
 pass to the ruined volcanoes of St. Paul, Fernando de 
 Noronha, Ascension, St. Helena, Trinidad and Tristan 
 d'Acunha. From this great Atlantic band two branches 
 proceed to the eastward, one through Central Europe, 
 where all the vents are now extinct, and the other 
 
LENGTH OF THE VOLCANIC BANDS. 235 
 
 through the Mediterranean to Asia Minor, the great 
 majority of the volcanoes along the latter line being 
 now extinct, though a few are still active. The vol- 
 canoes on the eastern coast of Africa may be regarded 
 as situated on another branch from this Atlantic vol- 
 canic band. The number of active volcanoes on this 
 Atlantic band and its branches, exclusive of those in 
 the West Indies, does not exceed fifty. 
 
 From what has been said, it will be seen that, not 
 only do the volcanoes of the globe usually assume a 
 linear arrangement, but nearly the whole of them can 
 be shown to be thrown up along three well-marked 
 bands and the branches proceeding from them. The 
 first and most important of these bands is nearly 1 0,000 
 miles in length, and with its branches contains more' 
 than 1 50 active volcanoes ; the second is 8,000 miles 
 in length, and includes about 100 active volcanoes ; 
 the third is much more broken and interrupted, extends 
 to a length of nearly 1,000 miles, and contains about 
 50 active vents. The volcanoes of the eastern coast of 
 Africa, with Mauritius, Bourbon, Eodriguez, and the 
 vents along the line of the Eed Sea, may be regarded 
 as forming a fourth and subordinate band. 
 
 Thus we see that the surface of the globe is covered 
 by a network of volcanic bands, all of which traverse it 
 in sinuous lines with a general north-and-south direc- 
 tion, giving off branches which often run for hundreds 
 of miles, and sometimes appear to form a connection 
 between the great bands. 
 
236 VOLCANOES. 
 
 These four bands of volcanic vents, running in a 
 general north-and-south direction, separate four un- 
 equal areas within which the exhibitions of volcanic 
 activity are feeble or quite unknown. The two grandest 
 of the bands of volcanic activity, with their branches, 
 form an almost complete series encircling the largest 
 of the oceans. 
 
 To this rule of the linear arrangement of the vol- 
 canic vents of the globe and their accumulation along 
 certain well-marked bands, there are two very striking 
 exceptions, which we must now proceed to notice. 
 
 In the very centre of the continent formed by 
 Europe and Asia, the largest unbroken land-mass of 
 the globe, there rises from the great central plateau 
 the remarkable volcanoes of the Thian Shan Range. 
 The existence of these volcanoes, of which only obscure 
 traditional accounts had reached Europe before the 
 year 1858, appears to be completely established by the 
 researches of the Eussian traveller Semenof. Three 
 volcanic vents appear to exist in this region : the active 
 volcanoes of Boschan and Turfan or Hot-schen, and the 
 solfatara of Urumtsi. At a point situated about half- 
 way between these three volcanoes and the sea, another 
 active vent, that of Ujung-Holdongi, is said to exist. 
 Other volcanic phenomena have been stated to occur in 
 the great plateau of Central Asia, but the existence of 
 some at least of these appears to rest on very doubtful 
 evidence. The only accounts which we have of the 
 eruptions of these Thian Shan volcanoes are contained 
 
EXCEPTIONALLY-SITUATED VOLCANOES. 237 
 
 in Chinese histories and treatises on geography; and a 
 great service would be rendered to science could they 
 be visited by some competent explorer. 
 
 The second exceptionally-situated volcanic group is 
 that of the Sandwich Islands. While the Thian Shan 
 volcanoes rise in the centre of the largest unbroken 
 land-mass, and stand on the edge of the loftiest and 
 greatest plateau in the world, the volcanoes of the Sand- 
 wich Islands rise almost in the centre of the largest ocean 
 and from almost the greatest depths in that ocean. 
 All round the Sandwich Islands the sea has a depth of 
 from 2,000 to 3,000 fathoms, and the island-group cul- 
 minates in several volcanic cones which rise to the 
 height of nearly 14,000 feet above the sea-level. The 
 volcanoes of the Sandwich Islands are unsurpassed in 
 height and bulk by those of any other part of the 
 globe. 
 
 With the exception of the two isolated groups of 
 the Thian Shan and the Sandwich Islands, nearly all 
 the active volcanoes of the globe are situated near the 
 limits which separate the great land- and water-masses 
 of the globe that is to say, they occur either on the 
 parts of continents not far removed from their coast- 
 lines, or on islands in the ocean not very distant from 
 the shores. 
 
 The fact of the general proximity of volcanoes to 
 the sea, is one which has frequently been pointed out 
 by geographers, and may now be regarded as being 
 thoroughly established. Even the apparently anoma- 
 
238 VOLCANOES. 
 
 lous case of the Thian Shan volcanoes is susceptible of 
 explanation if we remember the fact, now well ascer- 
 tained by geological researches, that as late certainly as 
 Pliocene times, a great inland sea spread over the 
 districts where the Caspian, the Sea of Aral, and many 
 other isolated lakes are now found. Upon the southern 
 shore of this sea rose the volcanoes of the Thian Shan, 
 some of which have not yet fallen into a state of com- 
 plete extinction. 
 
 But although the facts concerning the general 
 proximity of volcanoes to the ocean may be admitted 
 to be thoroughly established, yet inferences are some- 
 times hastily drawn from these facts which the latter, 
 if fairly considered, will not be found to warrant. It 
 is frequently assumed that we may refer all the remark- 
 able phenomena of volcanic action to the penetration 
 of sea- water to a mass of incandescent lava in the 
 earth's crust, and to the chemical or mechanical action 
 which would result from this meeting of sea-water and 
 molten rock. And this conclusion is supposed to find 
 support in the circumstance that many of the gases 
 and volatile' substances emitted from volcanic vents are 
 such as would be produced by the decomposition of the 
 various salts contained in sea-water. 
 
 This argument in favour of the production of vol- 
 canic outbursts by the irruption of sea-water into sub- 
 terranean reservoirs, involves, as Mr. Scrope long ago 
 pointed out, a curious example of reasoning in a circle. 
 It is assumed, on the one hand, that the heaving sub- 
 
CAUSE OF PROXIMITY OF VOLCANOES TO SEA. 239 
 
 terranean movements, which give rise to the fissures by 
 which steam and other gases escape to the surface, 
 are the result of the passage of water to heated masses 
 in the earth's crust. But, on the other hand, it is sup- 
 posed that it is the production of these fissures which 
 leads to the influx of water to the heated materials. If 
 it is the passage of water through these fissures which 
 produces the eruptions, it may be fairly asked, what is 
 it that gives rise to the fissures ? And if, on the other 
 hand, there exist subterranean forces competent to pro- 
 duce the fissures, may they not also give rise to the erup- 
 tions through the openings which they have originated? 
 Nor does the chemical argument appear to rest upon 
 any surer ground. It is true that many of the volatile 
 substances emitted from volcanic vents are such as 
 might be produced by the decomposition of sea-water, 
 but, upon the other hand, there are not a few substances 
 which cannot possibly be regarded as so produced, and 
 all the materials may equally well be supposed to have 
 been originally imprisoned in the masses of subterra- 
 nean lava. 
 
 The problem before us is this. Granting that it is 
 proved that active volcanoes are always in close prox- 
 imity to the ocean, are we to explain the fact by sup- 
 posing that the agency of sea-water is necessary to 
 volcanic outbursts, or by regarding the position of the 
 coast-lines as to some extent determined by the dis- 
 tribution of volcanic action upon the surface of the 
 globe ? The first supposition is the one which perhaps 
 
240 VOLCANOES. 
 
 most readily suggests itself, but the latter, as we shall 
 hereafter show, is one in favour of which not a few 
 weighty arguments may be advanced. 
 
 Another problem which suggests itself in connection 
 with the distribution of volcanoes is the following. Are 
 the great depressed tracts which form the bottom of 
 the oceans, like the elevated tracts which constitute 
 the continents, equally free from exhibitions of volcanic 
 energy ? 
 
 When we remember the fact that the area of the 
 ocean beds is two and three-quarter times as great as 
 that of the continents, it will be seen how important 
 this question of the existence of volcanoes at the bot- 
 tom of the ocean really is. 
 
 The fact that recent deep-sea soundings have shown 
 the deepest parts of the ocean to be everywhere covered 
 with volcanic debris is by no means conclusive upon this 
 question ; for, as we have seen, the ejections of sub- 
 aerial volcanoes are by the wind and waves distributed 
 over every part of the earth's surface. 
 
 Submarine volcanic outbursts have occurred in 
 many parts of the globe, but it may well be doubted 
 whether any such outburst has ever commenced at the 
 bottom of a deep ocean, and has succeeded in building 
 up a volcanic cone reaching to the surface. Most, if 
 not all, of the recorded submarine outbursts have 
 occurred in the midst of volcanic districts, and the 
 volcanic cones have been built up in water of no 
 great depth. Indeed, when it is remembered that 
 
SUBMAKINE ERUPTIONS. 241 
 
 the pressure of each 1,000 fathoms of water is equiva- 
 lent to a weight of more than one ton on every square 
 inch of the ocean-bottom, it is difficult to imagine the 
 ordinary explosive action of volcanic vents taking place 
 at abysmal depths. If, however, fissures were opened 
 in the beds of the ocean, quiet outwellings of lava 
 might possibly occur. 
 
 The solution of this problem of the probable 
 existence of volcanic outbursts on the floor of the 
 ocean can only be hoped for from the researches of 
 the geologist. The small specimens of the ocean-beds 
 brought up by deep-sea sounding-lines, taken at wide 
 distances apart, and including but a few inches from 
 the surface, can certainly afford but little information 
 upon the question. But the geologist has the op- 
 portunity of studying the sea-bottoms of various 
 geological periods which have been upheaved and are 
 now exposed to his view. It was at one time supposed 
 by geologists that in the so called i trap-rocks ' we 
 have great lava-sheets which must have been piled 
 upon one another, without explosive action. But the 
 more accurate researches of recent years have shown 
 that between the layers of ' trap-rock,' in every part of 
 the globe, traces of terrestrial surfaces and freshwater 
 deposits are found ; and the supposed proofs of the 
 absence of explosive action break down no less signally 
 upon re-examination ; for the loose, scoriaceous materials 
 would either be removed by denudation, or converted 
 into hard and solid rocks by the infilling of their 
 
242 VOLCANOES. 
 
 vesicles and air-cavities with crystalline minerals. It 
 is not possible, among the representatives of former 
 geological periods, to point to any rocks that can be 
 fairly regarded as having issued from great submarine 
 fissures, and it is therefore fair to conclude that no 
 such great outbursts of the volcanic forces take place 
 at the present day on the deep ocean-floors. 
 
 In connection with the question of the relation 
 between the position of the volcanic bands of the 
 globe and the areas covered by the ocean, we may 
 mention a fact which deep-sea soundings appear to 
 indicate, namely, that the deepest holes in the ocean- 
 floor are situated in volcanic areas. Near Japan, the 
 soundings of the U.S. ship < Tuscarora ' showed that at 
 two points the depth exceeded 4,000 fathoms; and 
 the deepest sounding obtained by H.M S. ' Challenger,' 
 amounting to 4,575 fathoms, was taken in the voyage 
 from New Guinea to Japan, in the neighbourhood of 
 the Ladrone Islands. Depths nearly as great were 
 found in the soundings carried on in the neighbour- 
 hood of the volcanic group of the West Indian Islands. 
 It must be remembered, however, that at present our 
 knowledge of the depths of the abysmal portions of 
 the ocean is very limited. A few lines of soundings, 
 often taken at great distances apart, are all we have to 
 guide us to any conclusions concerning the floors of 
 the great oceans, and between these lines are enormous 
 areas which still remain altogether unexplored. It 
 may be wise, therefore, to suspend our judgment upon 
 
KELATIONS TO MOUNTAIN-CHAINS. 243 
 
 such questions till more numerous facts have been 
 obtained. 
 
 Another fact concerning the distribution of vol- 
 canoes which is worthy of remark is their relation to 
 the great mountain-ranges of the globe. 
 
 Many of the grandest mountain-chains have bands 
 of volcanoes lying parallel to them. This is strikingly 
 exhibited by the great mountain-masses which lie 
 on the western side of the American continent. The 
 Kocky Mountains and the Andes consist of folded and 
 crumpled masses of altered strata which, by the action 
 of denuding forces, have been carved into series of 
 ridges and summits. At many points, however, along 
 the sides of these great chains, we find that fissures 
 have been opened and lines of volcanoes formed, from 
 which enormous quantities of lava have flowed and 
 covered great tracts of country. At some parts of the 
 chain, however, the volcanoes are of such height and 
 dimensions as to overlook and dwarf the mountain- 
 ranges by the side of which th >y lie. Some of the 
 volcanoes lying parallel to the great American axis 
 appear to be quite extinct, while others are in full 
 activity. 
 
 In the Eastern continent we find still more striking 
 examples of the parallelism between great mountain- 
 chains and the lands along which volcanic activity is 
 exhibited. Stretching in a more or less continuous 
 chain from east to west, through Europe and Asia, we 
 find the mountain-masses known in different parts of 
 
244 VOLCANOES. 
 
 their course as the Pyrenees, the Alps, the Balkan, 
 the Caucasus, which form the axis of the Eastern 
 continent. These chains consist of numerous parallel 
 ridges, and give off branches on either side of them. 
 They are continued to the eastward by the Hindoo 
 Koosh and the Himalaya, with the four parallel ranges 
 that cross the .great Central-Asian plateau. Now, on 
 either side of this grand axial system of mountains, we 
 find a great parallel band of volcanoes. The northern 
 volcanic band is constituted by the eruptive rocks of the 
 Auvergne, the Eifel, the Siebengebirge, Central Ger- 
 many, Bohemia, Hungary, and Transylvania, few, if any, 
 of the vents along this northern band being still active. 
 The remarkable volcanoes of the Thian Shan range 
 and of Mantchouria may not improbably be regarded 
 as a continuation of the same great series. 
 
 The southern band of volcanoes, lying parallel to 
 the great mountain axis of the Old World, also consists 
 for the most part of extinct volcanoes, but includes 
 not a few vents which are still active. In this band 
 we include the extinct volcanoes of Spain and Sardinia, 
 the numerous extinct and active vents of the Italian 
 peninsula and islands, and those of the ^Egean Sea and 
 Asia Minor. We may, perhaps, consider the scattered 
 volcanoes of Arabia and the northern part of the Indian 
 Ocean as a continuation of the same series. Both ol 
 these b&,ads may be regarded as offshoots from the 
 great mid- Atlantic volcanic chain, and the condition of 
 the vents, both in the principal band and its offshoots, 
 
RELATION TO AEEAS OF UPHEAVAL. 246 
 
 is such as to indicate that they form parts of a system 
 which is gradually sinking into a state of complete 
 extinction. 
 
 There are some other volcanic bands which exhibit 
 a similar parallelism with mountain chains ; but, on the 
 other hand, there are some volcanoes between which 
 and the nearest mountain axes no such connection can 
 be traced. 
 
 There is yet one other fact concerning the mode of 
 distribution of volcanoes upon the surface of the globe, 
 to which we must allude. It was first established by 
 Mr. Darwin as one of the conclusions derived from the 
 valuable series of observations made by him during the 
 voyage of H.M.S. < Beagle,' and relates to the position 
 of active volcanoes with respect to the portions of the 
 earth's crust which are undergoing upheaval or sub- 
 sidence. 
 
 From the relative position of the different kinds of 
 coral-reefs, and the fact that reef-building corals cannot 
 live at a depth of more than twenty fathoms beneath 
 the sea-level, or above tide-mark, we are led to the con- 
 clusion that certain areas of the earth's surface are un- 
 dergoing slow elevation, while other parts are as gradu- 
 ally subsiding. This conclusion is confirmed by the 
 occurrence of raised beaches, which are sometimes found 
 at heights of hundreds, or even thousands, of feet above 
 the sea-level, and of submerged forests, which are not 
 unfrequently found beneath the waters of the ocean. 
 
 By a study of the evidences presented by coral- 
 
246 VOLCANOES. 
 
 reefs, raised beaches, submerged forests, and other 
 phenomena of a similar kind, it can be shown that cer- 
 tain wide areas of the land and of the ocean-floor are at 
 the present time in a state of subsidence, while other 
 equally large areas are being upheaved. And the ob- 
 servations of the geologist prove that similar upward 
 and downward movements of portions of the earth's 
 crust have been going on through all geological times. 
 Now, as Mr. Darwin has so well shown in his work 
 on * Coral-Reefs,' if we trace upon a map the areas of 
 the earth's surface which are undergoing upheaval and 
 subsidence respectively, we shall find that nearly all 
 the active volcanoes of the globe are situated upon 
 rising areas, and that volcanic phenomena are con- 
 spicuously absent from those parts of the earth's crust 
 which can be proved at the present day to be under- 
 going depression. 
 
CHAPTER IX. 
 
 VOLCANIC ACTION AT DIFFERENT PERIODS OF THE 
 
 IT is only in comparatively recent times that the im- 
 portant doctrine of geological continuity has come to 
 be generally accepted, as furnishing us with a complete 
 and satisfactory explanation of the mode of origin of 
 the features of our globe. The great forces, which are 
 ever at work producing modifications in those features, 
 operate so silently and slowly, though withal so surely, 
 that without the closest and most attentive observation 
 their effects may be easily overlooked ; while, on the 
 other hand, there are so many phenomena upon our globe 
 which seem at first sight to bear testimony to the action 
 of sudden and catastrophic forces, very different to any 
 which appear to be at present at work, that the tendency 
 to account for all past changes by these violent actions 
 is a very strong one. In spite of this tendency, how- 
 ever, the real potency of the forces now at work upon 
 the earth's crust has gradually made its way to recog- 
 nition, and the capability of these forces, when their 
 effects are accumulated through sufficiently long periods 
 
248 VOLCANOES. 
 
 of time, to bring about the grandest changes, is now 
 almost universally admitted. The modern science oi 
 geology is based upon the principle that the history of 
 the formation and development of the earth's surface- 
 features, and the organisms upon it, has been continuous 
 during enormous periods of time, and that in the study 
 of the operations taking place upon the earth at the 
 present day, we may find the true key to the changes 
 which have occurred during former periods. 
 
 In no branch of geological science has the doctrine 
 of continuity had to encounter so much opposition and 
 misconception as in that which relates to the volcanic 
 phenomena of the globe. For a long time students of 
 rocks utterly failed to recognise any relation between 
 the materials which have been ejected from active 
 volcanic vents and those which have been formed by 
 similar agencies at earlier periods of the earth's history. 
 And what was far worse, the subject became removed 
 from the sphere of practical scientific inquiry to that of 
 theological controversy, those who maintained the vol- 
 canic origin of some of the older rocks being branded 
 as the worst of heretics. 
 
 With the theological aspects of the great controversy 
 concerning the origin of basalt and similar rocks a 
 controversy which was carried on with such violence 
 and acrimony during the latter half of the eighteenth 
 century we have here nothing to do. But it may not 
 be uninstructive to notice the causes of the strange 
 misconceptions which for so long a period stood in the 
 
CONTROVERSY CONCERNING ORIGIN OF BASALT. 249 
 
 way of the acceptance of rational views upon the sub- 
 ject. 
 
 At this period but little had been done in studying 
 the chemical characters of aqueous and igneous rock- 
 rnasses respectively ; and while, on the one hand, the 
 close similarity in chemical composition between the 
 ancient basalts and many modern lavas was not recog- 
 nised, the marked distinction between the composi- 
 tion of such materials and most aqueous sediments 
 remained, on the other hand, equally unknown. Nor 
 had anything been yet accomplished in the direction 
 of the study of rock-masses by the aid of the micro- 
 scope. Hence there could be no appeal to those 
 numerous structural peculiarities that at once enable 
 us to distinguish the most crystalline aqueous rocks 
 from the materials of igneous origin. 
 
 On the other hand, there undoubtedly exist rocks 
 of a black colour and crystalline structure, sometimes 
 presenting a striking similarity in general appearance 
 to the basalts, which contain fossils and are undoubt- 
 edly of aqueous origin. Thus on the shore near 
 Portrush, in the North of Ireland, and in the skerries 
 which lie off that coast, there occur great rock-masses, 
 some of which undoubtedly agree with basalt in all 
 their characters, while others are dark-coloured and 
 crystalline, and are frequently crowded with Ammonites 
 and other fossils. We now know that the explanation 
 of these facts is as follows. Near where the town of 
 Portrush is now situated, a volcanic vent was opened 
 12 
 
250 VOLCANOES. 
 
 in Miocene times through rocks of Lias shale. From 
 this igneous centre, sheets and dykes of basaltic lava 
 were given off, and in consequence of their contact with 
 these masses of lava, the Lias shales were baked and 
 altered, and assumed a crystalline character, though 
 the traces of the fossils contained in them were not 
 altogether obliterated. In the last century the methods 
 which had been devised for the discrimination of rocks 
 were so imperfect that no distinction was recognised 
 between the true basalt and the altered shale, and spe- 
 cimens of the latter containing Ammonites found their 
 way to almost every museum in Europe, and were used 
 as illustrations of the ' origin of basalt by aqueous pre- 
 cipitation/ 
 
 Another source of the widely-spread error which 
 prevailed concerning the origin of basalt, was the failure 
 to recognise the nature of the alterations which take 
 place in the character of rock-masses in consequence of 
 the passage through them, during enormous periods of 
 time, of water containing carbonic acid and other active 
 chemical agents. The casual observer does not recog- 
 nise the resemblance which exists between certain 
 ornamental marbles and the loose accumulations of 
 shells and corals which form many sea-beaches ; but 
 close examination shows that the former consist of 
 the same materials as the latter, bound together by a 
 crystalline infilling of carbonate of lime, which has 
 been deposited in all the cavities and interstices of the 
 mass. In the same way, as we have already seen, the 
 
VOLCANIC OEIGIN OF ' TEAP KOCKS.' 251 
 
 vesicles and interstices of heaps of scoriae may, by the 
 percolation of water through the mass, become so filled 
 with various crystalline substances, that its original 
 characters are entirely masked. 
 
 But the progress of chemical and microscopic re- 
 search has effectually removed these sources of error. 
 Many rocks of aqueous origin, formerly confounded 
 with the basalts, have now been relegated to their 
 proper places among the different classes of rocks; 
 while, on the other hand, it has been shown that the 
 chemical and physical differences between the ancient 
 basalts and the modern basic lavas are slight and acci- 
 dental, and their resemblances are of the closest and 
 most fundamental character. 
 
 The notion of the aqueous origin of basalt, which 
 was so long maintained by the school of Werner, has 
 now been entirely abandoned, and the so-called ' trap- 
 rocks ' are at the present day recognised as being as 
 truly volcanic in their origin as the lavas of Etna and 
 Vesuvius. 
 
 There is, however, a vestige of this doctrine of 
 Werner, which still maintains its ground with obstinate 
 persistence. Many geologists in Germany who admit 
 that volcanic phenomena, similar to those which are 
 going on at the present day, must have occurred during 
 the Tertiary and the later Secondary periods, neverthe- 
 less insist that among the earlier records of the world's 
 history we find no evidence whatever of such volcanic 
 action having taken place. By the geologists who hold 
 
252 VOLCANOES. 
 
 these views it is asserted that while the granites and 
 other plutonic rocks were formed during the earlier 
 periods of the world's history, true volcanic products are 
 only known in connection with the sediment of the 
 later geological periods. 
 
 Some geologists have gone farther even than this, 
 and asserted that each of the great geological periods 
 is characterised by the nature of the igneous ejections 
 which have taken place in it.' They declare that granite 
 was formed only during the earliest geological periods, 
 and that at later dates the gabbros, diabases, porphyries, 
 dolerites and basalts, successively made their appear- 
 ance, and finally that the modern lavas were poured 
 out. 
 
 A little consideration will suffice to convince us 
 that these conclusions are not based upon any good 
 evidence. The plutonic rocks, as we have already seen, 
 exhibit sufficient proofs in their highly crystalline cha- 
 racter, and in their cavities containing water, liquefied 
 carbonic acid, and other volatile substances, that they 
 must have been formed by the very slow consolidation 
 of igneous materials under enormous pressure. Such 
 pressures, it is evident, could only exist at great depths 
 beneath the earth's surface. Mr. Sorby and others have 
 endeavoured to calculate what was the actual thickness 
 of rock under which certain granites must have been 
 formed, by measuring the amount of contraction in 
 the liquids which have been imprisoned in the crystals 
 of these rocks. The conclusions arrived at are of a 
 
ANCIENT AND MODEKN VOLCANIC KOCKS. 253 
 
 sufficiently startling character. It is inferred that the 
 granites which have been thus examined must have 
 consolidated at depths varying from 30,000 to 80,000 
 feet beneath the earth's surface. It is true that in 
 arriving at these results certain assumptions have to 
 be made, and to these exception may be taken, but the 
 general conclusion that granitic rocks could only have 
 been formed under such high pressures as exist at 
 great depths beneath the surface, appears to be one 
 which is not open to reasonable doubt. 
 
 If, then, granites and similar rocks were formed at 
 the depth of some miles, it is evident that they can 
 only have made their appearance at the surface by the 
 removal of the vast thickness of overlying rocks ; and 
 the sole agency which we know of that is capable of 
 effecting the removal of such enormous quantities of 
 rock-materials, is denudation. But the agents of denu- 
 dation rain and frost, rivers and glaciers, and sea-waves 
 though producing grand results, yet work exceeding 
 slowly ; and almost inconceivably long periods of time 
 must have elapsed before masses of rock several miles 
 in thickness could have been removed, and the sub- 
 jacent granites and other highly crystalline rocks have 
 been exposed at the surface. 
 
 It is an admitted fact that among the older geological 
 formations, we much more frequently find intrusions of 
 granitic rocks than in the case of younger ones. It is 
 equally true that among the sediments formed during 
 the most recent geological periods, no true granitic rocks 
 
254 VOLCANOES. 
 
 have been detected. But if, as we insist is the case, 
 granitic rocks can only be formed at a great depth from 
 the surface, the facts we have described are only just 
 what we might expect to present themselves under the 
 circumstances. The older a mass of granitic rock, the 
 greater chance there is that the denuding forces operat- 
 ing upon the overlying masses, will have had an oppor- 
 tunity of so far removing the latter as to expose the 
 underlying crystalline rocks at the surface. And, on 
 the other hand, the younger crystalline rocks are still, 
 for the most part, buried under such enormous thick- 
 nesses of superincumbent materials that it is hopeless 
 for us to search for them. Nevertheless, it does occa- 
 sionally happen that, where the work of denudation 
 has been exceptionally rapid in its action, such crystal- 
 line rocks formed during a comparatively recent geo- 
 logical period, are exposed at the surface. This is the 
 case in the Western Isles of Scotland and in the 
 Pyrenees, where masses of granite and other highly 
 crystalline rocks are found which were evidently formed 
 during the Tertiary period. 
 
 The granites which were formed in Tertiary times 
 present no essential points of difference from those 
 which had their origin during the earlier periods of the 
 earth's history. The former, like the latter, consist of 
 a mass of crystals with no imperfectly crystalline base 
 or ground-mass between them ; and these crystals in- 
 clude numerous cavities containing liquids. 
 
 Between the granites and the quartz-felsites every 
 
RELATIONS BETWEEN GRANITE AND PUMICE. 255 
 
 possible gradation may be found, so that it is impossible 
 to say where the one group ends and the other begins ; 
 indeed, many of the rocks called ' granite-porphyries ' 
 have about equal claims to be placed in either class. 
 Nor is the distinction between the quartz-felsites and 
 rhyolites any more strongly marked than that between 
 the former class of rocks and the granites; some of 
 the more crystalline rhyolites of Hungary being quite 
 undistinguishable, in their chemical composition, their 
 mineralogical constitution, and their microscopic cha- 
 racters, from the quartz-felsites. The more crystalline 
 rhyolites are in turn found passing by insensible grada- 
 tions into the glassy varieties and finally into obsidian. 
 A piece of granite and a piece of pumice may at 
 first sight appear to present so many points of difference, 
 that it would seem quite futile to attempt to discover 
 any connection between them. Yet, if we analyse the 
 two substances, we may find that in ultimate chemical 
 composition they are absolutely identical. There is 
 nothing irrational, therefore, in the conclusion that the 
 same materials under different conditions may assume 
 either the characters of granite on the one hand, or 
 of pumice on the other ; the former being consolidated 
 under circumstances in which the chemical and crys- 
 talline forces have had the freest play and have used 
 up the whole of the materials to form crystallised 
 minerals, while the latter has cooled down and solidi- 
 fied rapidly at the surface, in such a way that only in- 
 cipient crystallisation has occurred, and the glassy 
 
256 VOLCANOES. 
 
 mass has been reduced to a frothy condition by the 
 escape of steam-bubbles from its midst. This con- 
 clusion receives the strongest support from the fact 
 that examples of every stage of the change, between 
 the glassy condition of pumice and the crystalline con- 
 dition of granite, may be detected among the materials 
 of which the globe is built up. 
 
 There is still another class of facts which may be 
 adduced in support of the same conclusion. Many 
 lavas, as we have seen, contain crystals of much larger 
 dimensions than those constituting the mass of the 
 rock, which is then said to be * porphyritic ' in struc- 
 ture. The porphyritically embedded crystals, when 
 carefully examined, are often seen to be broken and 
 injured, and to exhibit rounded edges,- with other in- 
 dications of having undergone transport. When ex- 
 amined microscopically, too, they often present the 
 cavities containing liquids which distinguish the crys- 
 tals of plutonic rocks. All the facts connected with 
 these porphyritic lavas point to the conclusion that 
 while the crystals in their ground-mass have separated 
 from the liquefied materials near the surface, the large 
 embedded crystals have been floated up from great 
 depths within the earth's crust, where they had been 
 originally formed. 
 
 The careful consideration of all the facts of the case 
 leads to the conclusion that where pumice, obsidian, 
 and rhyolite are now being ejected at the surface, the 
 materials which form these substances are 3> at various 
 
GRANITIC REPKESENTATIVES OF OTHEK LAVAS. 257 
 
 depths in the earth's interior, slowly consolidating in the 
 form of quartz-felsite, granite-porphyry and granite. It 
 may be that we can nowhere point to the example of a 
 mass of rock which can be traced from subterranean 
 regions to the surface, and is, under such conditions, 
 actually seen -to pass from the dense and crystalline 
 condition of granite to the vesicular and glassy form of 
 pumice ; but great granitic masses often exhibit a more 
 coarsely crystalline condition in their interior, and the 
 offshoots and dykes whifh they give off not unfre- 
 quently assume the form of quartz-felsite ; while, on 
 the other hand, the more slowly consolidated rocks 
 found in the interior of some rhyolite masses are not 
 distinguishable in any way from some of the true 
 quartz-felsites. 
 
 That which is true of the lavas of acid composition 
 is equally true of the lavas of intermediate and basic 
 character. The andesites, the trachytes, the phono- 
 lites, and the basalts have all their exact representatives 
 among the plutonic rocks, and these have a perfectly 
 crystalline or granitic structure. The plutonic and the 
 volcanic representatives of each of these groups are 
 identical in their chemical composition, and numerous 
 intermediate gradations can be found between the 
 most completely granitic and the most perfectly 
 vitreous or glassy types. In illustration of this fact, 
 we may again refer to the series of microscopic sections 
 of rocks given in the frontispiece. 
 
 Another objection to the conclusion that the volcanic 
 
258 VOLCANOES. 
 
 products of earlier periods of the earth's history were 
 identical in character with those which are being ejected 
 at the present day is based on the fact of the supposed 
 non-existence of the scoriaceous and glassy materials 
 which abound in the neighbourhood of the active vol- 
 canic vents. Where, it is asked, do we find among the 
 older rocks of the globe the heaps of lapilli, dust, and 
 scoriae, with the glassy and pumiceous rocks that now 
 occur so abundantly in all volcanic districts ? 
 
 In reply to this objection, we may point out that 
 these accumulations of loose materials are of such a 
 nature as to be capable of easy removal by denuding 
 agents, and that as they are formed upon the land they 
 will, if not already washed away by the action of rain, 
 floods, rivers, &c., run great risk of having their mate- 
 rials distributed, when the land sinks beneath the 
 waters of the ocean and the surface is covered by new 
 deposits. With respect to the glassy rocks it must be 
 remembered that the action of water containing carbonic 
 acid and other substances through such masses has a 
 tendency to set up crystalline action> and these glassy 
 rocks easily undergo c devitrification ' ; it would there- 
 fore be illogical for us to expect glassy rock-masses to 
 retain their vitreous character through long geologi- 
 cal periods, during which they have been subjected to 
 the action of water and acid gases. 
 
 But careful observation has shown that the scori- 
 aceous and vitreous rocks are by no means absent 
 among the igneous materials ejected during earlier 
 
SEMILAEITY OF ANCIENT AND KECENT LAVAS. 259 
 
 periods of the earth's history. Their comparative in- 
 frequency is easily accounted for when we remember, 
 in the first place, the ease with which such materials 
 would be removed by denuding forces, and in the 
 second place, the tendency of the action of percolating 
 water to destroy their characteristic features, by filling 
 up their vesicles with crystalline products and by 
 effecting devitrification in their mass. 
 
 If we go back to the very oldest known rock-masses 
 of the globe, those which are found underlying the 
 fossiliferous Cambrian strata, we find abundant evidence 
 that volcanic action took place during the period in 
 w r hich these materials were being accumulated. Thus, 
 in the Wrekin, as Mr. Allport has so well shown, we find 
 clear proofs that before the long-distant period of the 
 Cambrian, there existed volcanoes which ejected scoria3, 
 lapilli, and volcanic dust, and also gave rise to streams 
 of lava exhibiting the characteristic structures found 
 in glassy rocks. In these rocks, which have undergone 
 a curious alteration or devitrification, we still find all 
 those peculiar structures the sphaerulitic, the perlitic, 
 and the banded so common in the rhyolites of Hun- 
 gary, with which rocks the Wrekin lavas, in their 
 chemical composition, precisely agree. Prof. Bonney, 
 too, has shown that the rocks of Charnwood Forest, 
 which are also probably of pre-Cambrian age, contain 
 great quantities of altered volcanic agglomerates, tuffs, 
 and ashes. I have found the sphserulitic, perlitic, and 
 banded structures exhibited by British lavas of the 
 
260 VOLCANOES. 
 
 Cambrian, Silurian, Devonian and Carboniferous pe- 
 riods, as well as in those of Tertiary age ; and in 
 connection with these different lavas we find vast ac- 
 cumulations, sometimes thousands of feet in thickness, 
 of volcanic agglomerates and tuffs which have under- 
 gone great alteration. 
 
 All these facts point to one conclusion namely, 
 that during all past geological periods, materials similar 
 to those which are now being extruded from volcanic 
 vents were poured out on the earth's surface by analo- 
 gous agencies. If we could trace the lava-streams of 
 the present day down to the great subterranean re- 
 servoirs from which their materials have been derived, 
 we should doubtless find that at gradually increasing 
 depths, where the pressure would be greater and the 
 escape of heat from the mass slower, the rocky ma- 
 terials would by degrees assume more and more crystal- 
 line characters. We should thus find obsidian or rhyo- 
 lite insensibly passing into quartz-felsite and finally 
 into granite ; trachyte passing into orthoclase-porphyry 
 and syenite ; and basalt passing into dolerite, augite- 
 porphyry, and gabbro. 
 
 On the other hand, if we could replace the great 
 masses of stratified rocks which must once have overlain 
 the granites, syenites, diorites, and gabbros, we should 
 find that, as we approached the original surface, these 
 igneous materials would gradually lose their crystalline 
 characters, and when they were poured out at the sur- 
 face would take the forms of rhyolite, trachyte, andesite, 
 
ALTERED FORMS OF ANCIENT LAVAS. 261 
 
 and basalt all of which might occasionally assume the 
 glassy forms known as obsidian or tachylyte. 
 
 But while we insist on the essential points of simi- 
 larity between the lavas poured out upon the surface of 
 the earth during earlier geological periods and those 
 which are being extruded at the present day, we must 
 not forget that by the action of percolating water and 
 acid gases, the mineral constitution, the structure, and 
 sometimes even the chemical composition of these 
 ancient lavas may undergo a vast amount of change. 
 In not a few cases these changes in the characters of a 
 lava may be carried so far that the altered rock bears 
 but little resemblance to the lava from which it was 
 formed, and it may be found desirable to give it a new 
 name. Among the rocks of aqueous origin we find 
 similar differences in the materials deposited at different 
 geological periods. Clay, shale and clay-slate have the 
 same composition, and the two latter are evidently only 
 altered forms of the first mentioned, yet so great is the 
 difference in their characters that it is not only allow- 
 able, but desirable, to give them distinctive names. 
 
 In the same way, among the deposits of the earlier 
 geological periods we find rocks which were doubtless 
 originally basalts, but in which great alterations have 
 been produced by the percolation of water through the 
 mass. The original rock has consisted of crystals of 
 felspar, augite, olivine, and magnetite distributed 
 through a glassy base. But the chemical action of 
 water and carbonic acid may have affected all the in- 
 
262 VOLCANOES. 
 
 gredients of the rock. The outward form of the fel- 
 spar crystals may be retained while their substance 
 is changed to kaolinite, various zeolites, and other mine- 
 rals ; the olivine may be altered to serpentine and other 
 analogous minerals ; the magnetite changed to hydrous 
 peroxide of iron ; the augite may be changed to uralite 
 or hornblende ; and the surrounding glassy mass more 
 or less de vitrified and decomposed. The hard, dense, 
 and black rock known as basalt has under these cir- 
 cumstances become a much softer, earthy-looking mass 
 of a reddish-brown tint, and its difference from basalt 
 is so marked that geologists have agreed to call it by 
 another name, that of { melaphyre.' Even in their ul- 
 timate chemical compositions the c melaphyres ' differ 
 to some extent from the basalts, for some of the 
 materials of the latter may have been removed in 
 solution, and water, oxygen, and carbonic acid have 
 been introduced to combine with the remaining ingre- 
 dients. 
 
 But if we carefully study, by the aid of the micro- 
 scope, a large series of basalts and melaphyres, we shall 
 find that many rocks of the former class show the first 
 incipient traces of those changes which would reduce 
 them to the latter class. Indeed, it is quite easy to 
 form a perfect series from quite unaltered basalts to 
 the most completely changed melaphyres. Hence we 
 are justified in concluding that all the melaphyres were 
 originally basalts, just as we infer that all oaks were 
 once acorns. 
 
NAMES GIVEN TO ALTEKED LAVAS. 263 
 
 Now changes, similar to those which we have seen 
 to take place in the case of basaltic lavas, are exhibited 
 by the lavas of every other class, which have been ex- 
 posed to the influence of the same agencies, namely, 
 the passage of water and acid gases. But inasmuch as 
 the minerals composing the basic lavas are for the most 
 part much more easily affected by such agencies than 
 are the minerals of acid lavas, the ancient basic rocks are 
 usually found in a much more highly altered condition 
 than are the acid rocks of equivalent age. 
 
 We thus see that each of the classes of modern 
 lavas has its representative in earlier geological periods, 
 in the form of rocks which have evidently been derived 
 from these lavas through alterations effected by the 
 agency of water and acid-gases that have permeated 
 their mass. Thus, while the basalts are represented 
 among the ancient geological formation by the mela- 
 phyres, the andesites are represented by the porphy- 
 rites, and the trachytes and rhyolites by different 
 varieties of felstones. And, as we can form perfect 
 series illustrating the gradual change from basalt to 
 melaphyre, so we can arrange other series demonstrat- 
 ing the passage of andesites into porphyrites, and of 
 trachytes and rhyolites into felsites. 
 
 It must be remembered, however, that these changes 
 do not take place in anything like determinate periods 
 of time. Occasionally we may find lavas of very ancient 
 date which have undergone surprisingly little alteration, 
 and in other cases there occur lavas belonging to a 
 
264 VOLCANOES. 
 
 comparatively recent period which exhibit remarkably 
 few signs of change. 
 
 The alteration of the lavas and other igneous rocks 
 does not, however, stop with the production of the 
 melaphyres, porphyrites, and felstones. By the further 
 action of the water and carbonic acid of the atmosphere, 
 the basic lavas are reduced to the soft earthy mass 
 known as * wacke,' and the intermediate and acid lavas 
 to the similar material known as claystone.' As the 
 passage of water and carbonic acid gas through these 
 rock-masses goes on, they are eventually resolved into 
 two portions, one of which is insoluble in water and 
 the other is soluble. The insoluble portion consists 
 principally of quartz, the crystals of which are almost 
 unattacked by water and carbonic acid, and the hydrated 
 silicate of alumina. All the sands and clays, which 
 together make up more than nine-tenths of the strati- 
 fied rocks of the globe, are doubtless derived, either 
 directly or indirectly, from these insoluble materials 
 separated during the decomposition of volcanic and 
 plutonic rocks. The soluble materials, which consist 
 of the carbonates, sulphates and chlorides of lime, mag- 
 nesia, soda, potash, and iron, give rise to the formation 
 of the limestones, gypsum, rock-salt, ironstones, and 
 other stratified masses of the earth's crust. We thus 
 see how the igneous materials of the globe, by their 
 decomposition, furnish the materials for the stratified 
 rock-masses. The relations of the different plutonic 
 and volcanic rocks to one another and to the materials 
 
KELATIONS OF ALTERED TO UNALTERED LAVAS. 265 
 
 Unaltered 
 
 rolite and 
 >sidian 
 
 which are derived from them 
 following table. 
 
 Plutonic rocks 
 GRANITE . 
 
 SYENITE . 
 DIORITE . 
 
 MlASCITE . 
 
 GABBRO 
 
 are illustrated in the 
 
 {Quartz-i 
 (' quartz- 
 
 Altered lavas 
 
 Felstone 
 
 Poi-phyrite 
 
 Decomposed 
 rocks 
 
 Clavstones 
 
 Phonolite 
 
 f Liebnerite 
 t porphyry 
 TAugite-por- ~i 
 < phyry and y Basalt 
 (.DoleriU 
 
 
 Melaphyre Wacke 
 
 Some petrographers, indeed, have maintained the 
 principle that rocks belonging to widely separated geo- 
 logical periods, even when they exhibit no essential 
 points of difference, should nevertheless be called by 
 distinct names. But such a system of classification is 
 calculated rather to hinder than to advance the cause 
 of science. If the palaeontologist were to adopt the same 
 principle and give distinct names to the same fossil, 
 when it was found to occur in two different geological 
 formations, we can easily understand what confusion 
 would be occasioned, and how the comparison of the 
 fauna and flora of the different formations would be 
 thereby rendered impossible. But the naturalist, in his 
 diagnosis of a species, wisely confines himself to the 
 structure and affinities of the organism before him ; and 
 in the same way the petrographer, in giving a name to 
 a rock, ought to be guided only by his studies of its 
 chemical composition, its mineralogical constitution, 
 and its structure, putting altogether out of view its 
 
266 VOLCANOES. 
 
 geographical distribution and geological age. Only by 
 strict attention to this principle can we hope to arrive 
 at such comparisons of the rocks of different areas and 
 different periods, as may serve as the basis for safe 
 inductions. 
 
 Before leaving this question of the relation which 
 exists between the igneous rocks of different ages, it 
 may be well to notice several facts that have been 
 relied upon, as proving that the several geological 
 periods are distinguished by characteristic igneous 
 products. 
 
 It has frequently been asserted that the acid igneous 
 rocks are present in much greater quantities in connec- 
 tion with the older geological formations than are the 
 basic; while, on the other hand, the basic igneous 
 rocks are said to have been extruded in greater abund- 
 ance in the more recent geological periods. But in con*- 
 sidering this question it must not be forgotten that, 
 as a general rule, the basic rocks undergo decomposi- 
 tion and disintegration far more rapidly than do the 
 acid rocks. In consequence of this circumstance the 
 chance of our finding their recognisable representatives 
 among the older formations, is much less in the case 
 of the former class of rocks than in the latter. As a 
 matter of fact, however, we do find great masses of 
 gabbro, diabase, and melaphyre associated even with 
 the oldest geological formations, while trachytes and 
 rhyolites abound in many volcanic districts where ac- 
 tive vents exist at the present day. Upon a general 
 
r THE 
 : .R 
 OF 
 
 r> c. 
 
 UNIVERSITY 
 
 AUGITIC AND HOENBLENDIC ROCKS. 267 
 
 review of the subject, it may well be doubted whether 
 the supposed preponderance of acid igneous materials 
 in the earlier periods of the earth's history, and of 
 basic igneous materials during the later periods, rests 
 on any substantial basis of observation. 
 
 Another difference which has frequently been relied 
 upon, as distinguishing the older igneous rocks from 
 those of more recent date, is the supposed fact that the 
 former are characterised by the presence of hornblende, 
 the latter by the presence of augite. It may be ad- 
 mitted that this distinction is a real one, but its 
 significance and value are greatly diminished when we 
 remember the relations which exist between the two 
 minerals in question. Hornblende and augite are 
 interesting examples of a dimorphous substance; in 
 chemical composition they are identical, or rather they 
 are liable to variation between the same limits, but in 
 their crystalline forms and optical characters they differ 
 from one another. It has been proved that hornblende is 
 the stable, and augite the unstable condition of the sub- 
 stance in question. If hornblende be fused and allowed 
 to cool, it crystallises in the form of augite. On the 
 other hand, augite-crystals in rocks of ancient.date are 
 found undergoing gradual change and passing into 
 hornblende. The mineral uralite has the outward form 
 of augite, but the cleavage and optical properties of 
 hornblende ; and there are not wanting many facts 
 pointing to the conclusion that rocks which now con- 
 tain hornblende were originally augitic masses, in which 
 
268 VOLCANOES. 
 
 the unstable mineral in their midst has been gradually 
 converted into the stable one. 
 
 There are, however, two minerals which up to the 
 present time have been found in association only with 
 the older and newer rock- masses respectively. These 
 are muscovite, or the white form of mica, which occurs 
 in so many granites, but has not yet been discovered 
 in any modern representative of that rock ; and leucite, 
 which is not yet known in rocks of older date than the 
 Tertiary. 
 
 When we remember that muscovite would appear 
 to be a product of deep-seated igneous action, and is 
 only found in rock-masses that have been formed under 
 such conditions, we shall be the less surprised at its 
 non- occurrence in rocks of recent date, especially if we 
 bear in mind the fact that very few of the younger 
 granitic rocks have as yet been exposed at the surface 
 by denudation. 
 
 With respect to leucite, on the other hand, it must 
 be remembered that it is a very unstable mineral which 
 appears to be easily changed into felspar. It is by no 
 means improbable, therefore, that some ancient igneous 
 rocks which now contain felspar were originally leucitic 
 rocks. 
 
 To the view that the action of volcanic forces upon 
 the globe during past geological times was similar in 
 kind to that which we now observe going on around us, 
 still another objection has been raised. It has been 
 asserted that some of the deposits of igneous rock 
 
VOLCANIC ORIGIN OF ANCIENT IGNEOUS EOCKS. 269 
 
 associated with the older geological formations are of 
 such a nature that they could not possibly have been 
 accumulated around volcanic vents of the kind which 
 we see in operation around us. 
 
 Mr. Mallet has declared that the igneous products 
 of the Palaeozoic period differ fundamentally in charac- 
 ter from those materials formed by volcanic action 
 during the later Secondary and the Tertiary periods. 
 Upon what observations these generalisations are based 
 he has given us no information, and the enormous 
 mass of facts which have been collected in recent years 
 concerning the structure of the lavas and fragmental 
 volcanic deposits of the pre-Cambrian, Cambrian, Si- 
 lurian, Devonian and Carboniferous periods, all point 
 to a directly opposite conclusion. The more carefully 
 we carry on our investigations concerning these ancient 
 lavas, by the aid of chemical analysis and microscopic 
 study, the more are we convinced of the essential 
 identity of the ancient and modern volcanic rocks, 
 both in their composition and their minute structure. 
 Of great masses of dust produced by crushing, such as 
 Mr. Mallet has supposed to have been formed during 
 the earlier geological periods, there is not the smallest 
 evidence ; but we everywhere find proofs, when the 
 rocks are minutely examined, of the vesicular structure 
 so characteristic of materials produced by explosive 
 volcanic action. 
 
 It has frequently been asserted that in the great 
 districts covered by basaltic lavas which we find in the 
 
270 VOLCANOES. 
 
 JRocky Mountains of North America, in the Deccan of 
 India, in Abyssinia, and even in the Western Isles of 
 Scotland, we have proofs of the occurrence, during 
 earlier geological periods, of volcanic action very differ- 
 ent in character from that which at present takes place 
 on our globe. It has been asserted that the phenomena 
 observed in these districts can only be accounted for 
 by supposing that great fissures have opened in their 
 midst, from which lavas have issued in enormous floods 
 unaccompanied by the ordinary explosive phenomena 
 of volcanoes. 
 
 It must be remembered, however, that none of the 
 districts in question have been subjected to careful 
 and systematic examination with a view to the discovery 
 of the vents from which these masses of lava have 
 issued, with the exception of that which occurs in our 
 own islands. In this case, in which superficial observers 
 have spoken of the district as being covered with hori- 
 zontal lava-sheets piled upon one another to the 
 depth of 3,000 feet, careful study of the rock-masses 
 has shown that the accumulations of basalt really con- 
 sist of a great number of lava-currents which have 
 issued at successive epochs covering enormous periods 
 of time. During the intervals between the emission 
 of these successive lava-currents the surfaces of the 
 older ones have been decomposed, and formed soils 
 upon which forests have grown up ; they have been 
 eroded by streams, the valleys so formed being filled 
 with gravels ; and lakes have been originated on their 
 
TKULY-VOLCANIC OKIGIN OF LAVA PLATEAUX. 271 
 
 surfaces in which various accumulations have taken 
 place. It has been demonstrated, moreover, that the 
 basal-wrecks of no less than five volcanic mountains, 
 each of which must have rivalled Etna in its propor- 
 tions, existed within this area, and the connection of 
 the lava-currents, which have deluged the surrounding 
 tracts, with these great volcanoes has been clearly proved. 
 It is probable that when more careful and systematic 
 researches are carried on in the other districts, in which 
 widely-spread sheets of basaltic rocks exist, similar 
 volcanic vents will be discovered. It must also be 
 remembered that if such a country as Iceland were 
 subjected to long-continued denudation, the mountain 
 peaks and cones of loose materials would be worn away, 
 the whole island being thus reduced to a series of 
 plateaux composed of lava-sheets, the connection of 
 which with the crystalline materials filling the great 
 volcanic vents, a superficial observer might altogether 
 fail to recognise. 
 
 But even where we cannot trace the former existence 
 of great volcanic mountains, 'like those which once rose 
 in the Hebrides, it would nevertheless be very rash to 
 conclude that the vast plateaux of lava-rock must have 
 been formed as gigantic floods unaccompanied by ordi- 
 nary volcanic action. Mr. Darwin has pointed out that 
 in crossing districts covered by lava, he was frequently 
 only able to determine the limits of the different cur- 
 rents of which it was made up, by an examination of 
 the age of the trees and the nature of the vegetation 
 
272 VOLCANOES. 
 
 which had sprung up on them. And everyone who 
 has travelled much in volcanic districts can confirm 
 this observation; what appears at first sight to be a 
 great continuous sheet of lava proves upon more care- 
 ful observation to be composed of a great number of 
 distinctly different lava-currents, which have succeeded 
 one another at longer or shorter intervals. 
 
 We must remember, too, how various in kind are 
 the volcanic manifestations which present themselves 
 under different circumstances. Sometimes the amount 
 of explosive action at a volcanic vent is very great, and 
 only fragmental ejections take place, composed of the 
 frothy scum of the lava produced by the escape of gases 
 and vapours from its midst. But in other cases the 
 amount of explosive action may be small, and great 
 volumes of igneous materials may issue as lava-streams. 
 In such cases, only small scoria-cones would be formed 
 around the vents, and one half of such cones is com- 
 monly swept away by the efflux of the lava-currents, 
 while the remainder may be easily removed by denud- 
 ing action or be buried under the lava-currents issu- 
 ing from other vents in the neighbourhood. Thus it 
 may easily come to pass that what a superficial observer 
 takes for an enormous mass of basaltic lava poured out 
 from a great fissure at a single effort, may prove upon 
 careful observation to be made up of innumerable lava- 
 currents, each of which is of moderate dimensions ; and 
 it may further be found that these lava-currents, in- 
 stead of being the product of a single paroxysmal effort 
 
SHIFTING OF VOLCANIC ACTION INDIFFERENT AKE AS. 273 
 
 from one great fissure, have been accumulated by nu- 
 merous small outbursts at wide intervals, from a great 
 number of minor orifices. 
 
 Having then considered the arguments which have 
 been adduced in support of the view that the volcanic 
 phenomena of former geological periods differ from 
 those which are still occurring upon the globe, we may 
 proceed to state the general conclusions which have been 
 drawn from the study of the volcanic rocks of the dif- 
 ferent geological periods. 
 
 From a survey of the volcanic rocks of different ages, 
 we are led to the interesting and important conclusion 
 that the scene of volcanic action has been continu- 
 ally shifting to fresh areas at different periods of the 
 earth's history. We find repeated proofs that the 
 volcanic energy has made its appearance at a certain 
 part of the earth's crust, has gradually increased in 
 intensity to a maximum, and then as slowly declined. 
 But as these manifestations have died away at one part 
 of the earth's surface, they have gradually made their 
 appearance at another. In every district which has 
 been examined, we find abundant proofs that volcanic 
 energy has been developed at certain periods, has dis- 
 appeared during longer or shorter periods, and then 
 reappeared in the same area. And on the other hand, 
 we find that there is no past geological period in which 
 we have not abundant evidence that volcanic outbursts 
 took place at some portion of the earth's surface. 
 
 To take the case of our own islands for example. 
 13 
 
274 VOLCANOES. 
 
 We know that during the pre-Cambrian periods volcanic 
 outbursts took place, traces of which are found both in 
 North and South Wales, in the Wrekin Chain in Shrop- 
 shire, in Charawood Forest, and in parts of Scotland 
 and Ireland. 
 
 In Cambro-Silurian times we have abundant proofs, 
 both in North Wales and the Lake district, that vol- 
 canic action on the very grandest scale was taking place 
 during the Arenig and the older portion of the Llandeilo 
 periods, and again during the deposition of the Bala 
 or Caradoc beds. The lavas, tuffs, and volcanic agglo- 
 merates ejected during these two periods have built up 
 masses of rock many thousands of feet in thickness. 
 Snowdon and Cader Idris among the Welsh mountains, 
 and some of the higher summits of the Lake district, 
 have been carved by denudation from the vast piles of 
 volcanic materials ejected during these periods. 
 
 In Devonian or Old-Eed-Sandstone times, volcanic 
 activity was renewed with fresh violence upon that 
 part of the earth's surface now occupied by the British 
 Islands. Along the line which now forms the Gram- 
 pians there rose a series of volcanoes of the very 
 grandest dimensions. Ben Nevis, and many others 
 among the higher Scotch mountains, have been carved 
 by denudation from the hard masses of granite, quartz- 
 felsite, and other plutonic rocks which formed the cen- 
 tral cores of these ancient volcanic piles. The remains 
 of the great lava-sheets, and of the masses of volcanic 
 agglomerate ejected from these grand Devonian vol- 
 
ANCIENT BRITISH VOLCANOES. 275 
 
 canoes, make up hill-ranges of no mean altitude, like 
 the Sidlaws, the Ochils, and the Pentlands. 
 
 The volcanic action of the Devonian period was pro- 
 longed into Carboniferous times, but was then evidently 
 gradually diminishing in violence. Instead of great 
 central volcanoes, such as existed in the earlier period, 
 we find innumerable small vents which threw out tuffs, 
 agglomerates and lavas, and were scattered over the 
 districts lying around the bases of the now extinct 
 Devonian volcanoes. In the central valley of Scotland 
 and in many parts of England, we find abundant proofs 
 of the existence of these small and scattered volcanic 
 vents during Carboniferous times. The well-known hill 
 of Arthur's Seat, which overlooks the city of Edinburgh, 
 and many castle-crowned crags of the Forth and Clyde 
 valleys, are the worn and denuded relics of 'these small 
 volcanoes. There are some indications which point to 
 the conclusion that the volcanic action of the Newer 
 Palaeozoic epoch had not entirely died out in Permian 
 times, but the evidence upon this point is not altogether 
 clear and satisfactory. 
 
 During nearly the whole of the Secondary or Meso- 
 zoic periods the volcanic forces remained dormant in 
 the area of the British Isles. Some small volcanic out- 
 bursts, however, appear to have occurred in Triassic 
 times in Devonshire. But in other areas, such as the 
 Tyrol, South-eastern Europe and Western America, the 
 Triassic, Jurassic, and Cretaceous periods were marked 
 by grand manifestations of volcanic activity. 
 
276 VOLCANOES. 
 
 . The volcanic forces which had during the long 
 Mesozoic periods deserted our part of the earth's sur- 
 face, appear to have returned to it in full vigour in the 
 Tertiary epoch. In the Newer-Palaeozoic periods the 
 direction of the great volcanic band which traversed 
 our islands appears to have been from north-east to 
 south-west ; but in Tertiary times a new set of fissures 
 were opened running from north to south. There is 
 evidence that during the Eocene or Nummulitic period, 
 the first indications of the subterranean forces having 
 gathered strength below the district were afforded by 
 the issue of calcareous and siliceous springs, and soon 
 fissures were opened which emitted scoriae, tuffs, and 
 lavas. The intensity of the volcanic action gradually 
 increased till it attained its maximum in the Miocene 
 period, when a great chain of volcanic mountains 
 stretched north and south along the line of the Inner 
 Hebrides, the north-east of Ireland, and the sea which 
 separates Great Britain from Ireland. The basal- 
 wrecks of a number of these volcanoes can be tra,ced 
 in the islands of Skye, Mull, Eum, and parts of the 
 adjoining mainland. We have already seen that along 
 this great band of volcanic action, which traverses the 
 Atlantic Ocean from north to south, a number of active 
 vents still exist, though their energy is now far less 
 intense than was the case in former times. The only 
 vestiges of the action of these now declining volcanic 
 forces, at present found in our islands, are the hot 
 springs of Bath and a few other warm and mineral 
 
ANCIENT VOLCANOES IN OTHEE DISTRICTS. 277 
 
 springs; but in connection with this subject it must be 
 remembered that our country occasionally participates 
 in great earthquake-vibrations, like that which de- 
 stroyed Lisbon in the year 1759. 
 
 If we were to study any other part of the earth's 
 surface, we should arrive at precisely the same con- 
 clusion as those to which we have been conducted by 
 our examination of the British Islands namely, that 
 during past geological times the subterranean forces 
 had made themselves felt in the area, had gradually 
 attained a maximum, and then as gradually declined, 
 passing through all those varied cycles which we have 
 described in a former chapter. And we should also 
 find that these periods of volcanic activity alternated 
 with other periods of complete quiescence which were 
 of longer or shorter duration. But on comparing two 
 different districts, we should discover that what was a 
 period of volcanic activity in the one was a period of 
 repose in the other, and vice versa. 
 
 From these facts geologists have been led to the 
 conclusion which we have already enunciated namely, 
 that the subterranean forces are in a state of continual 
 flux over the surface o f the globe. At one point of the 
 earth's crust these forces gradually gather such energy 
 as to rend asunder the superincumbent rock-masses 
 and make themselves manifest at the surface in the 
 series of phenomena characteristic of volcanic action. 
 But after a longer or shorter interval of time an in- 
 terval which must probably be measured by millions of 
 
278 VOLCANOES. 
 
 years the volcanic forces die out in that area to make 
 their appearance in another. 
 
 Hence, although we may not be able to prove the 
 fact by any mathematical demonstration, a strong pre- 
 sumption is raised in favour of the view that the 
 subterranean energy in the earth's crust is a constant 
 quantity, and that the only variations which take place 
 are in the locality of its manifestation. 
 
 This leads us to the question whether the amount 
 of this subterranean energy within the earth's crust 
 is at the present time increasing, stationary, or de- 
 clining. There are some considerations connected with 
 certain astronomical hypotheses, to which we shall 
 hereafter have to refer, that might lead us to entertain 
 the view that the subterranean activity was once far 
 greater than it is at present, and that during the long 
 periods of the earth's past history it has been slowly 
 and gradually declining. And those who examine the 
 vast masses of igneous materials which have been 
 poured out from volcanic vents during the earlier 
 periods of the earth's history may be inclined, at first 
 sight, to point to them as affording conclusive proof of 
 this gradual decline. 
 
 But a more careful study of the rocks in question 
 will probably cause a geologist to pause before jumping 
 to such a conclusion. If we look at the vast masses of 
 volcanic materials erupted in Miocene times in our own 
 island and in Ireland, for example, we might be led 
 to imagine that we have the indications of a veritable 
 
SUPPOSED DECLINE OF VOLCANIC ACTION. 279 
 
 ' Reign of Fire,' and that the evidence points to a 
 condition of things very different indeed from that 
 which prevails at the present day. But in arriving at 
 such a conclusion we should be neglecting a most im- 
 portant consideration, the disregard of which has been 
 the fertile parent of many geological errors. Many 
 independent lines of evidence all point to the inference 
 that these volcanic ejections are not the result of one 
 violent effort, but are the product of numerous small 
 outbreaks which have been scattered over enormous 
 periods of time. 
 
 When we examine with due care the lavas, tuffs, 
 and other volcanic ejections which constitute such 
 mountain-masses as those of the Hebrides, of the Au- 
 vergne, and of Hungary, we find clear proofs that the 
 ancient Miocene volcanoes of these districts were clothed 
 with luxuriant forests, through which wild animals 
 roamed in the greatest abundance. The intervals be- 
 tween the ejections of successive lava-streams were 
 often so great, that soils were formed on the mountain- 
 slope, and streams cut deep ravines and valleys in 
 them. 
 
 The island of Java is situated near the very heart 
 of what is at the present day the most active volcanic 
 centre on the face of the globe, yet vegetable and 
 animal life nourish luxuriantly there, and the island 
 is one of the richest and most fertile spots upon the 
 face of the globe. Not all the terrors of occasional 
 volcanic outbreaks will ever drive the Neapolitan vine- 
 
280 VOLCANOES. 
 
 dressers from the fertile slopes of Vesuvius, for its 
 periods of repose are long, and its eruptions are of 
 short duration. 
 
 These considerations lead the geologist to conclude 
 that the evidence afforded by the ancient volcanic rocks 
 is clear and positive in support of the view that the 
 manifestations of the subterranean forces in the past 
 agree precisely in their nature and in their products 
 with those taking place around us at the present time. 
 On the question of great secular changes having 
 occurred in the amount of volcanic energy in past 
 geological periods, the evidence must be pronounced 
 negative, or at the best doubtful. 
 
 But even if the geologist confesses himself unable 
 to establish the fact of any decline in the subterranean 
 energies during the vast periods of which he takes 
 cognisance, it must be remembered that such decline 
 may really be going on ; for vast as was the duration of 
 the geological epochs, they probably constitute but a 
 fraction of those far grander periods which are required 
 by the speculations of the physical astronomer. 
 
CHAPTEK X. 
 
 THE PART PLAYED BY VOLCANOES IN THE ECONOMY OF 
 NATURE 
 
 THE first impression which is produced upon the mind, 
 when the phenomena of volcanic action are studied, is 
 that here we have exhibitions of destructive violence 
 the effects of which must be entirely mischievous and 
 disastrous to the living beings occupying the earth's 
 surface. A little consideration will convince us, how- 
 ever, that the grand and terrible character of the dis- 
 plays of volcanic energy have given rise to exaggerated 
 notions concerning their destructive effects. The fact 
 that districts situated over the most powerful volcanic 
 foci, like Java and Japan, are luxuriant in their pro- 
 ductions, and thickly inhabited, may well lead us to 
 pause ere we condemn volcanic action as productive only 
 of mischief to the living beings on the earth's surface. 
 The actual slopes of Vesuvius and Etna, and many other 
 active volcanoes, are abundantly clothed with vineyards 
 and forests and are thickly studded with populous 
 villages. 
 
 As a matter of fact, the actual amount of damage 
 
282 VOLCANOES. 
 
 to life and property which is effected by volcanic erup- 
 tions is small. Usually, the inhabitants of the district 
 have sufficient warning to enable them to escape with 
 their lives and to carry away their most valuable pos- 
 sessions. And though fertile tracts are covered by 
 sterile dust and ashes, or by lava- and mud-currents, 
 yet the sterility of the latter is generally of short dura- 
 tion, for by their decomposition such volcanic mate- 
 rials give rise to the formation of the richest and most 
 productive soils. 
 
 Earthquakes, as we have already seen, are far more 
 destructive in their effects than are volcanoes. Houses 
 and villages, nay even entire cities, are, by vibrations 
 of portions of the earth's crust, reduced to heaps of 
 ruins, and famines and pestilences too frequently follow, 
 as the consequence of the disorganisation of our social 
 systems by these terrible catastrophes. 
 
 It may well be doubted, however, whether the 
 annual average of destruction to life and property 
 caused by all kinds of subterranean action, exceeds that 
 produced either by floods or by hurricanes. Yet we 
 know that the circulation of water and air over our 
 globe are beneficial and necessary operations, and that 
 the mischief occasionally wrought by the moving bodies 
 of water and air is quite insignificant compared with 
 the good which they effect. 
 
 In the same way, we shall be able to show that the 
 subterranean energies are necessary to the continued 
 existence of our globe as a place fitted for the habita- 
 
LEVELLING ACTION OF DENUDING FORCES. 283 
 
 tion of living beings, and that the mischievous and 
 destructive effects of these energies bear but a small 
 and insignificant proportion to the beneficial results 
 with which they must be credited. 
 
 We have had frequent occasion in the preceding 
 pages to refer to the work slow but sure, silent 
 but effective wrought by the action of the denuding 
 forces ever operating upon the surface of our globe. The 
 waters condensing from the atmosphere and falling upon 
 the land in the form of rain, snow, or hail, are charged 
 with small quantities of dissolved gases, and these 
 waters penetrating among the rock-masses of which the 
 earth's crust is composed, give rise to various chemical 
 actions of which we have already noticed such remark- 
 able illustrations in studying the ancient volcanic pro- 
 ducts of our globe. By this action the hardest and 
 most solid rock-masses are reduced to a state of com- 
 plete disintegration, certain of their ingredients under- 
 going decomposition, and the cementing materials which 
 hold their particles together being removed in a state of 
 solution. In the higher regions of the atmosphere this 
 work of rock-disintegration proceeds with the greatest 
 rapidity; for there the chemical action is reinforced by 
 the powerful mechanical action of freezing water. On 
 high mountain-peaks the work of breaking up rock- 
 masses goes on at the most rapid rate, and every craggy 
 pinnacle is swathed by the heaps of fragments which 
 have fallen from it. The Alpine traveller justly dreads 
 the continual fusillade of falling rock-fragments which 
 
284 VOLCANOES. 
 
 is kept up by the ever-active power of the frost in these 
 higher regions of the atmosphere ; and fears lest the 
 vibrations of his footsteps should loosen, from their 
 position of precarious rest, the rapidly accumulating 
 piles of detritus. No mountain-peak attains to any 
 very great elevation above the earth's surface, for the 
 higher we rise in the atmosphere the greater is the 
 range of temperature and the more destructive are the 
 effects of the atmospheric water. The moon, which ie 
 a much smaller planet than our earth, has mountains 
 of far greater elevation ; but the moon possesses neither 
 an atmosphere nor moisture on its surface, to produce 
 those levelling effects which we see everywhere going 
 on around us upon the earth. 
 
 The disintegrated materials, produced by chemical 
 and mechanical actions of the atmospheric waters upon 
 rock-masses, are by floods, rivers, and glaciers, gradually 
 transported from higher to lower levels ; and sooner 01 
 later every fragment, when it has once been separated 
 from a mountain-top, must reach the ocean, where these 
 materials are accumulated and arranged to form new 
 rocks. 
 
 Over every part of the earth's surface these three 
 grand operations of the disintegration of old rock- 
 masses, the transport of the materials so produced to 
 lower levels, and the accumulation of these materials to 
 form new rocks, is continually going on. It is by the 
 varied action of these denuding agents upon rocks of 
 unequal hardness, occupying different positions in re- 
 
NECESSITY EOK COMPENSATING AGENCIES. 285 
 
 lation to one another, that all the external features of 
 hills, and plains, and mountains owe their origin. 
 
 It is a fact, which is capable of mathematical de- 
 monstration, that by the action of these denuding 
 forces the surface of all the lands of the globe is being 
 gradually but surely lowered ; and this takes place at 
 such a rate that in a few millions of years the whole 
 of the existing continents must be washed away and 
 their materials distributed over the beds of the oceans. 
 
 It is evident that there exists some agency by which 
 this levelling action of the denuding forces of the globe 
 is compensated ; and a little consideration will show that 
 such compensating agency is found in the subterranean 
 forces ever at work within the earth's crust. The 
 effects of these subterranean forces which most power- 
 fully arrest our attention are volcanic outbursts and 
 earthquake shocks, but a careful study of the subject 
 proves that these are by no means the most important 
 of the results of the action of such forces. Exact ob- 
 servation has proved that almost every part of the 
 earth's surface is either rising or falling, and the striking 
 and destructive phenomena of volcanoes and earth- 
 quakes probably bear only the same relation to those 
 grand and useful actions of the subterranean forces, 
 which floods do to the system of circulating waters, and 
 hurricanes to the system of moving air-currents. 
 
 If we ride in a -well-appointed carriage with good 
 springs, upon a railway which is in excellent order, the 
 movement is almost imperceptible to us ; and the rate 
 
286 VOLCANOES. 
 
 of speed may be increased indefinitely, without making 
 itself apparent to our senses. The smallest impediment 
 to the evenness of the movement such as that pro- 
 duced by a small object placed upon the rails at once 
 makes itself felt by a violent jar and vibration. How 
 perfectly insensible we may be of the grandest and 
 most rapid movements is taught us by the facts demon- 
 strated by the astronomei . By the earth's daily rota- 
 tion, we are borne along at a rate which in some places 
 .amounts to over 1,000 miles an hour; and by its an- 
 nual revolution we are every hour transported through 
 a distance of 70,000 miles; yet concerning the fact 
 and direction of these movements we are wholly un- 
 conscious. 
 
 In the case both of the railway train and of our 
 planet, we can only establish the reality of the move- 
 ment, and its direction and rate, by means of observa- 
 tions upon external objects, which appear to us to have 
 a movement in the opposite direction. In the same 
 way we can only establish the fact of the movement of 
 portions of the earth's crust by noticing the changing 
 positions of parts of the earth's surface in relation to 
 the constant level of the ocean. When this is done we 
 find abundant proof that while some parts of the 
 earth's crust are rising, others are as undoubtedly 
 undergoing depression. 
 
 We shall be able to form some idea of the vastness 
 of the effects produced by the subterranean forces, by 
 a very simple consideration. It is certain that during 
 
POTENCY OF THE SUBTERRANEAN FORCES. 287 
 
 the enormous periods of time of which the records 
 have been discovered by the geologist, there have al- 
 ways been continents and oceans upon the earth's 
 surface, just as at present, and it is almost equally 
 certain that the proportions of the earth's surface occu- 
 pied by land and water respectively, have not varied 
 very widely from those which now prevail. But, at the 
 same time, it is an equally well-established fact that 
 the denuding forces ever at work upon the earth's 
 surface would have been competent to the removal of 
 existing continents many times over, in the vast 
 periods covered by geological records. Hence we are 
 driven to conclude that the subterranean movements 
 have in past times entirely compensated for the waste 
 produced by the denuding forces ever at work upon our 
 globe. But this is not all. The subterranean forces 
 not only produce upheaval; in a great many cases the 
 evidences of subsidence are as clear and conclusive as 
 are those of upheaval in others. Hence we are driven 
 to conclude that the forces producing upheaval of por- 
 tions of the earth's crust are sufficient, not only to 
 balance those producing subsidence, but also to com- 
 pensate for the destructive action of denuding agents 
 upon the land-masses of the globe. 
 
 It is only by a careful and attentive study and 
 calculation of the effects produced by the denuding 
 agents at work all around us, aided by an examination 
 of the enormous thicknesses of strata formed by the 
 action of such causes during past geological times, that 
 
288 VOLCANOES. 
 
 we are able to form any idea of the reality and vastness 
 of the agents of change which are ever operating to 
 modify the earth's external features. When we have 
 clearly realised the grand effects produced on the sur- 
 face of the globe by these external forces, through the 
 action of its investing atmosphere and circulating 
 waters, then, and only then, shall we be in a position 
 to estimate the far greater effects resulting from the 
 internal forces, of which the most striking, but not the 
 most important, results are seen in the production of 
 volcanic eruptions and earthquake-shocks. 
 
 Another series of facts which serve to convince the 
 geologist of the reality and potency of the forces ever 
 at work within the earth's crust, and the way in which 
 these have operated during past geological periods, is 
 found in the disturbed condition of many of the 
 stratified rock-masses of which it is composed. Such 
 stratified rock-masses, it is clear, must have been 
 originally deposited in a position of approximate 
 horizontality ; but they are now often found in inclined 
 and even vertical positions ; they are seen to be bent, 
 crumpled, puckered, and folded in the most remarkable 
 manner, and have not unfrequently been broken across 
 by dislocations c faults ' which have sometimes dis- 
 placed masses, originally in contact, to the extent of 
 thousands of feet. The slate-rocks of the globe, more- 
 over, bear witness to the fact that strata have been 
 subjected to the action of lateral compression of enor- 
 mous violence and vast duration ; while in the meta- 
 
EELATION TO CONTINENTAL MOVEMENTS. 289 
 
 morphic rocks we see the effects of still more extreme 
 mechanical strains, which have been in part trans- 
 formed into chemical action. No one who has not 
 studied the crushed, crumpled, fractured, and altered 
 condition of many of the sedimentary rocks of the 
 globe, can form the faintest idea of the enormous 
 effects of the internal forces which have been in opera- 
 tion within the earth's crust during earlier geological 
 periods. And it is only by such studies as these that 
 we at last learn to regard the earthquake and volcanic 
 phenomena of our globe, not as the grandest and most 
 important effects of these forces, but as their secon- 
 dary and accidental accompaniments. 'Volcanoes,' it 
 has been said, * are the safety-valves of the globe ; ' 
 and when we come to realise the real extent and na- 
 ture of the internal forces ceaselessly working in the 
 earth's crust we shall scarcely be disposed to regard 
 the simile as an overstrained one. 
 
 The first geologist who attempted to show the exact 
 relations existing between those subterranean forces 
 which cause the movements of continental masses of 
 land, and those more startling displays of energy which 
 are witnessed in volcanic outbursts, was the late Mr. 
 Poulett Scrope. At a somewhat later date Mr. Darwin, 
 in his remarkable paper { On the Connexion of certain 
 Volcanic Phenomena in South America, and on the 
 Formation of Mountain-chains and Volcanoes as the 
 effect of Continental Elevations,' threw much new and 
 important light upon the question. 
 
290 VOLCANOES. 
 
 While, on the one hand, we are led by recent geo- 
 logical investigations to reject the notions which were 
 formerly accepted, by which mountain-ranges were sup- 
 posed to be suddenly and violently upheaved by volcanic 
 forces, we are, on the other hand, driven to conclude 
 that without the action of these subterranean forces, 
 the irregularities which are exhibited on the earth's 
 surface could not have had any existence. 
 
 It is true that the actual forms of the mountain- 
 ranges are due directly to the action of denuding forces, 
 which have sculptured out from the rude rocky masses 
 all the varied outlines of peaks and crags, of ravines and 
 valleys. But it is none the less true that the determin- 
 ing causes which have directed and controlled all this 
 earth-sculpture, are found in the relative positions of 
 hard and soft masses of rock; but these rock-masses 
 have acquired their hardness and consistency, and have 
 assumed their present positions, in obedience to the 
 action of subterranean forces. Hence we see that though 
 the formation of mountain-ranges is proximately due to 
 the denuding forces, which have sculptured the earth's 
 surface, the primary cause for the existence of such 
 mountain-chains must be sought for in the fact that 
 subterranean forces have been at work, folding, crump- 
 ling, and hardening the soft sediments, and placing 
 them in such positions that, by the action of denuda- 
 tion, the more indurated portions are left standing as 
 mountain-masses above the general surface. 
 
 The old notion tha 4 mountain-chains are due to a 
 
FOEMATION OF MOUNTAIN-CHAINS. 291 
 
 vertical up thrust from below, finds but little support 
 when we come to study with due care the positions 
 of the rock-masses composing the earth's crust. On 
 the contrary, we find that mountain-ranges are usually 
 carved out of the crushed and crumpled edges of strata 
 which have along certain lines been influenced by 
 great mechanical strains, and subjected to more or less 
 induration and chemical alteration. When we com- 
 pare these folded and contorted portions of the strata 
 with those parts of the same beds which are not so 
 affected, we find the effects produced in the former are 
 not such as would result from an up thrust from below, 
 but from movements by which a tangential strain 
 would be brought about. If we imagine certain lines 
 of weakness to exist in the solid crust of the earth, 
 then any movements in the portions of the crust be- 
 tween these lines of weakness would cause crushing 
 and crumpling of the strata along the latter. 
 
 Recent investigations of Dana and other authors 
 have thrown much new light upon the question of the 
 mode of formation of mountain-chains, and the relation 
 between the movements by which they are produced 
 and the sudden and violent manifestations of force 
 ^witnessed in volcanic outbursts. We cannot, perhaps, 
 better illustrate this subject than by giving a sketch of 
 the series of operations to which the great Alpine chains 
 owe their origin. 
 
 There are good grounds for believing that the great 
 mountain-axis of Southern Europe, with its continuation 
 
292 VOLCANOES. 
 
 in Asia, had no existence during the earlier geological 
 periods. Indeed, it has been proved that all the higher 
 among the existing mountain-chains of the globe have 
 been almost entirely formed in Tertiary times. The 
 reason of this remarkable fact is not far to seek. So 
 rapid is the work of denudation in the higher regions 
 of the atmosphere, that the elevated crags and pin- 
 nacles are being broken up by the action of moisture 
 and frost at an exceedingly rapid rate. This fact is 
 attested by the existence of those enormous masses of 
 angular rock-fragments which are found lodged on 
 every vantage-ground among the mountain-summits, 
 as well as by the continually descending materials 
 which are borne by glaciers and mountain-torrents to 
 the valleys below. Where such a rate of disintegration 
 as this is maintained, no elevated mountain- crests could 
 exist through long geological periods. It is true we 
 find in all parts of the globe relics of many mountain- 
 chains which were formed before the Tertiary period ; 
 but these have by long-continued denudation been worn 
 down to c mere stumps.' Of such worn-down and de- 
 graded mountain- ranges we have examples in the Scan- 
 dinavian chains, and some of the low mountain-regions 
 of Central Europe and North America. 
 
 Let UG now proceed to illustrate this subject by 
 briefly sketching the history of that series of opera- 
 tions by which the great mountain-chains of the 
 Alpine system have been formed. 
 
 The first stage of that grand series of operations 
 
VOLCANIC FISSUKES OF PERMIAN PEKIOD. 293 
 
 appears from recent geological researches to have con- 
 sisted in the opening of a number of fissures running 
 along a line near to that at which, in a long sub- 
 sequent period, the elevation of the mountain-masses 
 took place. This betrayal of the existence of a line of 
 weakness in this part of the earth's crust occurred in 
 the Permian period, and from that time onward a series 
 of wonderful movements and changes have been going 
 forward, which have resulted in the production of the 
 Alpine chains as we now see them. 
 
 From the great fissures opened in Permian times 
 along this line of weakness, great quantities of lava, 
 scoriae, and tuff were poured out, and these accumu- 
 lated to form great volcanic mountains, which we can 
 now only study at a few isolated spots, as in the Tyrol, 
 Carinthia, and about Lake Lugano. Everywhere else, 
 these Permian volcanic rocks appear to be deeply buried 
 under the later-formed sediments, from which the 
 Alpine chains have been carved. Few and imperfect, 
 however, as are the exposures of these ancient rhyolite 
 and quartz-andesite lavas and agglomerates formed at 
 the close of the Palaeozoic epoch, their greatly denuded 
 relics form masses which are in places more than 9,000 
 feet in thickness. From this fact we are able to form 
 some slight idea of the scale upon which the volcanic 
 outbursts in question must have taken place during 
 Permian times. 
 
 The second stage in the series of operations by 
 which the Alpine chains have been formed, consisted 
 
294 VOLCANOES. 
 
 in a general sinking of the surface along that line of 
 weakness in the earth's crust, the existence of which 
 had been betrayed by the formation of fissures and 
 the eruption of volcanic rocks. We have already had 
 occasion to remark how frequently such subsidences 
 follow upon the extrusion of volcanic masses at any 
 part of the earth's surface ; and we have referred these 
 downward movements in part to the removal of support 
 from below the portion of the crust affected, and in part 
 to the weight of the materials piled upon its surface by 
 the volcanic forces. 
 
 The volcanic energy which had been manifested 
 with such violence during the Permian period, does not 
 appear to have died out altogether during the succeed- 
 ing Triassic period. A number of smaller volcanic vents 
 were opened from time to time, and from these, lavas, 
 tuffs, and agglomerates, chiefly of basic composition, 
 were poured out. The relics of these old Triassic vol- 
 canoes are found at many points along the Alpine 
 chain, but it is evident that the igneous forces were 
 gradually becoming exhausted during this period, and 
 before the close o" it they had fallen into a state of 
 complete extinction. 
 
 But the great subsidence which had commenced in the 
 Triassic period, along what was to become the future 
 line of the Alpine chain, was continued almost without 
 interruption during the Rhsetic, the Jurassic, the Ti- 
 thonian, the Neocomian, the Cretaceous and the Num- 
 mulitic periods. With respect to the strata formed 
 
FOKMATION OF ALPINE GEOSYNCLINAL. 295 
 
 during all these periods, it is found that their thick- 
 nesses, which away from the Alpine axis may be 
 measured by hundreds of feet, is along that axis in- 
 creased to thousands of feet. The united thickness 
 of sediments accumulated along this great line of sub- 
 sidence between the Permian and Nummulitic periods 
 probably exceeds 50,000 feet, or ten miles. The sub- 
 sidence appears to have been very slow and gradual, 
 but almost uninterrupted, and the deposition of sedi- 
 ments seems to have kept pace with the sinking of the 
 sea -bottom, a fact which is proved by the circumstance 
 that nearly the whole of these sediments were such 
 as must have been accumulated in comparatively 
 shallow water. 
 
 By the means we have described there was thus 
 formed a ; geosynclinal,' as geologists have called it, 
 that is, a trough-like hollow filled with masses of ab- 
 normally thickened sediments, which had been piled 
 one upon another during the long periods of time in 
 which almost uninterrupted subsidence was going on 
 along the Alpine line of weakness in the earth's crust. 
 In this way was brought together that enormous ac- 
 cumulation of materials from which the hard masses of 
 the Alpine chains were subsequently elaborated, and 
 out of which the mountain-peaks were eventually 
 carved by denudation. 
 
 The third stage in this grand work of mountain- 
 making commenced in the Oligocene period. It con- 
 sisted of a series of movements affecting the parts of 
 
296 VOLCANOES. 
 
 the earth's crust on either side of the line of weakness 
 which had first exhibited itself in Permian times. By 
 these movements a series of tangential strains were 
 produced, which resulted in the violent crushing, fold- 
 ing, and crumpling of the sedimentary materials com- 
 posing the geosynclinal. 
 
 One effect of this action was the violent flexure and 
 frequent fracture of these stratified masses, which are 
 now found in the Alpine regions assuming the most 
 abnormal and unexpected positions and relations to one 
 another. Sometimes the strata are found tortured and 
 twisted into the most complicated folds and puckerings ; 
 at others they are seen to be completely inverted, so 
 that the older beds are found lying upon the newer ; 
 and in others, again, great masses of strata have been 
 traversed by numerous fractures or faults, the rocks on 
 either side of which are displaced to the extent of thou- 
 sands of feet. 
 
 Another effect of the great lateral thrusts by which 
 the thick sedimentary masses of the geosynclinal were 
 being so violently disturbed, was the production of a 
 great amount of induration and chemical change in 
 these rocks. Masses of soft clay, of the age of that upon 
 which London is built, were by violent pressure reduced 
 to the condition of roofing-slate, similar to that of North 
 Wales. One of the most important discoveries of modern 
 times is that which has resulted in the recognition of 
 the fact of the mutual convertibility of different kinds 
 of energy. We now know that mechanical force may 
 
FORMATION OF ALPINE GEANTICLINAL. 297 
 
 be transformed into he/at-force or chemical force ; and 
 of such transformations we find abundant illustrations 
 in the crushed and crumpled rock-masses of the Alpine 
 chains. 
 
 Under the influence of these several kinds of force, 
 not only was extreme consolidation and induration pro- 
 duced among the rock-masses, but chemical affinity 
 and crystalline action had the fullest play among the 
 materials of which they were composed. In many 
 cases we find the originally soft muds, sands, and shell- 
 banks converted into the most highly crystalline rocks, 
 which retain their primary chemical composition, but 
 have entirely lost all their other original features. 
 
 To the mass of folded, crumpled, and altered strata, 
 formed from a geosynclinal by lateral pressure, geolo- 
 gists have given the name of a 4 geanticlinal.' The 
 formation of the Alpine geanticlinal was due to move- 
 ments which commenced in the Oligocene period, at- 
 tained their maximum in the Miocene, and appear to 
 have declined and almost altogether died out in the 
 Pliocene period. 
 
 The movements which resulted in the crushing and 
 crumpling of the thickened mass of sediments along the 
 Alpine line of weakness, also gave rise to the formation 
 of a series of fissures from which volcanic action took 
 place. These fissures were not, however, formed along 
 the original line of weakness, for this had been 
 strengthened and repaired by the deposition of ten- 
 miles' thickness of sediments upon it, but along new 
 14 
 
298 VOLCANOES. 
 
 fissures opened in directions parallel to the original 
 lines of weakness, and in areas where a much less 
 considerable amount of deposition had taken place 
 since Permian times. 
 
 We have abundant evidence that, just at the period 
 when those great movements were commencing which 
 resulted in the formation of the great Alpine and 
 Himalayan geanticlinal, earth-fissures were being 
 opened upon either side of the latter from which 
 volcanic outbursts took place. At the period when the 
 most violent mountain-forming movements occurred, 
 these fissures were in their most active condition, and 
 at this time two great volcanic belts stretched east 
 and west, on either side of, and parallel to, the great 
 Alpine chain. The Northern volcanic band was formed 
 by the numerous vents, now all extinct, in Auvergne, 
 Central Germany, Bohemia, and Hungary, and was 
 probably continued in the volcanoes of the Thian Shan 
 and Mantchouria. The Southern volcanic band was 
 formed by the numerous vents of the Iberian and 
 Italian peninsulas, and the islands of the Mediter- 
 ranean, and were continued to the eastward by those 
 of Asia Minor, Arabia, and the North Indian Ocean. 
 As the earth-movements which produced the geanti- 
 clinal died away, the volcanic energy along these 
 parallel volcanic bands died away at the same time. 
 In studying the geology of Central and Southern 
 Europe, no fact comes out more strikingly than that 
 of the synchronism between the earth-movements by 
 
SCULPTUKING- OF ALPS BY DENUDATION. 299 
 
 which the geanticlinal of the Alps was formed, and the 
 volcanic manifestations which were exhibited along 
 lines of fissure parallel to that geanticlinal. The 
 earth-movements and the volcanic outbursts both com- 
 menced in the Oligocene period, gradually attained 
 their maximum in the Miocene, and as slowly declined 
 in the Pliocene. 
 
 The fourth stage in the great work of mountain- 
 building in the case of the Alps consisted in the opera- 
 tion of the denuding forces, the disintegrating action 
 of rain and frost, the transporting action of rivers and 
 glaciers, by which the Alpine peaks were gradually 
 sculptured out of the indurated and altered masses con- 
 stituting the geanticlinal. The action of this fourth 
 stage went on to a great extent side by side with that 
 of the third stage. So soon as the earth-movements 
 had brought the submerged sedimentary masses of the 
 geosynclinal under the action of the surface tides and 
 currents of the ocean, marine denudation would com- 
 mence ; and, as the work of elevation went on, the 
 rock-masses would gradually be brought within the 
 reach of those more silently-working but far more 
 effective agents which are ever operating in the higher 
 regions of the atmosphere. It is impossible to say 
 what would have been the height of the Alpine chain 
 if the work of denudation had not to a great extent 
 kept pace with that of elevation. Only the harder and 
 more crystalline masses have for the most part escaped 
 destruction, and stand up in high craggy summits; 
 
300 VOLCANOES. 
 
 while flanking bills, like the well-known Rigi, are seen 
 to be composed of conglomerates thousands of feet in 
 thickness, composed of their disintegrated materials. 
 It is a remarkable fact, as showing how enormous was 
 the work of elevation during the formation of the ge- 
 anticlinal, that some of the youngest and least con- 
 solidated rocks of the Nummulitic period are still 
 found at a height of 11,000 feet in the Alps, and of 
 16,000 feet in the Himalaya. 
 
 From what has been said, it will be seen that 
 mountain-chains may be regarded as cicatrised wounds 
 in the earth's solid crust. A line of weakness first 
 betrays itself at a certain part of the earth's surface by 
 fissures, from which volcanic outbursts take place ; and 
 thus the position of the future mountain-chain is de- 
 termined. Next, subsidence during many millions of 
 years permits of the accumulation of the raw materials 
 out of which the mountain-range is to be formed ; sub- 
 sequent earth-movements cause these raw materials to 
 be elaborated into the hardest and most crystalline 
 rock-masses, and place them in elevated and favourable 
 positions ; and lastly, denudation sculptures from these 
 hardened rock-masses all the varied mountain forms. 
 Thus the work of mountain-making is not, as was 
 formerly supposed by geologists, the result of a simple 
 upheaving force, but is the outcome of a long and com 
 plicated series of operations. 
 
 The careful study of other mountain-chains, espe- 
 cially those of the American continent, has shown that 
 
ORIGIN OF OTHER MOUNTAIN-CHAINS. 301 
 
 the series of actions which we have described as oc- 
 curring in the Alps, took place in the same order in 
 the formation of all mountain-masses. It is doubtful 
 whether the line of weakness is always betrayed in 
 the first instance by the formation along its course of 
 volcanic fissures. But in all cases we have evidence of 
 the production of a geosynclinal, which is afterwards, 
 by lateral pressure, converted into a geanticlinal, and 
 from this the mountain-chains have been carved by 
 denudation. Professor Dana has shown that the geo- 
 synclinal of the Appalachian chain was made up of 
 sediments attaining a thickness of 40,000 feet, or eight 
 miles ; while Mr. Clarence King has shown that a part 
 of the geosynclinal of the Eocky Mountains was built 
 up of no less than 60,000 feet, or twelve miles of 
 strata. 
 
 It has thus been established that a very remarkable 
 relation exists between the forces by which continental 
 masses of land are raised and depressed, and mountain- 
 ranges have been developed along lines of weakness 
 separating such moving continental masses, and those 
 more sudden and striking manifestations of energy 
 which give rise to volcanic phenomena. It is in this 
 relation between the widespread subterranean energies 
 and the local development of the same forces at vol- 
 canic vents, that we must in all probability seek for 
 the explanation of those interesting peculiarities of 
 the distribution of volcanoes upon the face of the globe 
 which we have described in a former chapter. Tne 
 
302 VOLCANOES. 
 
 parallelism of volcanic bands to great mountain-chains 
 is thus easily accounted for ; and in the same way we 
 may probably explain the position of most volcanoes 
 with regard to coast- lines. We have already pointed 
 out the objections to the commonly-received view that 
 volcanoes depend for their supplies of water on the 
 proximity of the ocean. This proximity of the ocean 
 to volcanic vents we are thus inclined to regard, not 
 as the cause, but as the effect of the subterranean 
 action. The positions of both volcanoes and coast-lines 
 are determined by the limits of those great areas of the 
 earth's crust which are subjected to slow vertical move- 
 ments, often in opposite directions. 
 
 Terrible and striking, then, as are the phenomena 
 connected with volcanic action, such sudden and vio- 
 lent manifestations of the subterranean energy must 
 not be regarded as the only, or indeed the chief, effects 
 which they produce. The internal forces continually 
 at work within the earth's crust perform a series of 
 most important functions in connection with the eco- 
 nomy of the globe, and were the action of these forces 
 to die out, our planet would soon cease to be fit for the 
 habitation of living beings. 
 
 There is no fact which the geological student is 
 more constantly called upon to bear in mind than that 
 of the potency of seemingly insignificant causes which 
 continue in constant operation through long periods 
 of time. Indeed these small and almost unnoticed 
 agencies at work upon the earth's crust are often found, 
 
EFFECTS OF SLOW CONTINENTAL MOVEMENTS. 303 
 
 in the long run, to produce far grander effects than 
 those of which the action is much more striking and 
 obvious. It is to the silent and imperceptible action 
 of atmospheric moisture and frost that the disintegra- 
 tion of the solid rock-masses must be mainly ascribed ; 
 and the noisy cataract and ocean-billow produce effects 
 which are quite insignificant compared with those 
 which must be ascribed to the slight and almost un- 
 noticed forces. Great masses of limestone are built up 
 of the remains of microscopic organisms, while the 
 larger and higher life-forms contribute but little to the 
 great work of rock-building. 
 
 In the same way it is to the almost unnoticed 
 action of the subterranean forces in raising some vast 
 areas of the earth's crust, in depressing others, and in 
 bringing about the development of mountain-chains 
 between them, that we must ascribe a far more im- 
 portant part in the economy of our globe than to the 
 more conspicuous but less constant action of volcanoes. 
 
 A few simple considerations will serve to convince 
 us, not only of the beneficial effects of the action of 
 the subterranean energies within the earth's crust, but 
 of the absolute necessity of the continued operation of 
 those energies to the perpetuation of that set of con- 
 ditions by which our planet is fitted to be the habita- 
 tion of living beings. 
 
 We have already referred to the prodigious effects 
 which are constantly being produced around us by the 
 action of the external forces at work upon the globe. 
 
304 VOLCANOES. 
 
 The source of these external forces is found in the 
 movements and changes which are ever going on 
 within the aqueous and atmospheric media in which 
 the globe is enveloped. The circulation of the air, 
 influencing the circulation of the waters in the shape 
 of clouds, rain, snow, rivers, glaciers, and oceans, causes 
 the breaking up of even the hardest rock-masses, and 
 the continual removal of their disintegrated fragments 
 from higher to lower levels. This work goes on with 
 more or less regularity over every part of the land 
 raised above the level of the ocean, but the rate of 
 destruction in the higher regions of the atmosphere is 
 far more rapid than at lower levels. Hence the cir- 
 culating air and water of the globe are found to be 
 continually acting as levellers of the land-masses of the 
 earth. 
 
 It is by no means a difficult task to calculate the 
 approximate rate at which the various continents and 
 islands are being levelled down, and such calculations 
 prove that in a very few millions of years the existing 
 forces operating upon the earth's surface would reduce 
 the whole of the land-masses to the level of the ocean. 
 
 But a little consideration will convince us that the 
 circulation of the air and waters of the globe are them- 
 selves dependent upon the existence of those irregu- 
 larities of the land-surfaces which they are constantly 
 tending to destroy. Without elevated mountain ridges 
 the regular condensation of moisture, and its collection 
 and distribution in streams and rivers over every part 
 
CONTRAST BETWEEN THE EARTH AND MOON. 305 
 
 of the land surfaces, could not take place. Under these 
 circumstances the unchecked evaporation of the oceanic 
 waters would probably go on, till the proportion of water- 
 vapour increased to such an extent in the atmosphere 
 as effectually to destroy those nicely-balanced conditions 
 upon which the continued existence of both vegetable 
 and animal life depend. 
 
 But the repeated upward and downward move- 
 ments which have been shown to be going on in the 
 great land-masses of the globe, giving rise in turns to 
 those lateral thrusts and tangential strains to which 
 mountain-chains owe their formation, afford a perfect 
 compensation to the action of the external forces ever 
 operating upon the earth's surface. 
 
 If, however, the uncompensated effect of the ex- 
 ternal forces acting on the earth's crust is calculated 
 to bring about the destruction of those conditions upon 
 which the existence of life depends, the uncompensated 
 effect of the internal forces acting on the earth's crust 
 are fraught with at least equal dangers to those neces- 
 sary conditions. 
 
 In our nearest neighbour among the planets the 
 moon the telescope has revealed to us the existence 
 of a globe, in which the internal forces have not been 
 checked and controlled by the operation of any external 
 agencies for the moon appears to be destitute of both 
 atmosphere and water. r 
 
 Under these circumstances we find its surface, as we 
 might expect, to be composed of rocks which appear 
 
306 VOLCANOES. 
 
 to be entirely of igneous origin ; the mountain-masses, 
 unworn by rain or frost, river or glacier, being of most 
 prodigious dimensions as compared with those of our 
 own globe, while no features at all resembling valleys, 
 or plains, or alluvial flats are anywhere to be discerned 
 upon the lunar surface. 
 
 But by the admirable balancing of the external and 
 internal forces on our own globe, the conditions neces- 
 sary to animal and vegetable existence are almost con- 
 stantly maintained, and those interruptions of such 
 conditions, produced by hurricanes and floods, by volcanic 
 outbursts and earthquakes, may safely be regarded as 
 the insignificant accidents of what is, on the whole, a 
 very perfectly working piece of machinery. 
 
 The ancients loved to liken the earth to a living 
 being the macrocosm of which man was the puny 
 representative or microcosm ; and when we study the 
 well-adapted interplay of the forces at work upon the 
 earth's crust, both from within and without, the analogy 
 seems a scarcely strained one. In the macrocosm and 
 the microcosm alike, slight interferences with the regu- 
 lar functions occasionally take place, and both of them 
 exhibit the traces of a past evolution and the germs of 
 an eventual decay. 
 
CHAPTEE XT. 
 
 WHAT VOLCANOES TEACH US CONCERNING THE NATURE OF 
 THE EARTH'S- INTERIOR. 
 
 IN entering upon any speculations or enquiries concern- 
 ing the nature of the interior of our globe, it is neces- 
 sary before all things that we should clearly realise in 
 our minds how small and almost infinitesimal s that 
 part of the earth's mass which can be subjected to 
 direct examination. The distance from the surface to 
 the centre of our globe is nearly 4,000 miles, but the 
 deepest mines do not penetrate to much more than half 
 a mile from the surface, and the deepest borings fall far 
 short of a mile in depth. Sometimes, it is true, the 
 geologist finds means for drawing inferences as to the 
 nature of the rocks at depths of ten or fifteen miles 
 below the surface ; but the last-named depth must be 
 regarded as the utmost limit of that portion of our 
 globe which can be made the object of direct observa- 
 tion and study. This thin exterior film of the earth's 
 mass, which the geologist is able to investigate, we call 
 the ' crust of the globe ' ; but it must be remembered 
 that in using this term, it is not intended to imply that 
 
308 VOLCANOES. 
 
 the outer part of our globe differs in any essential re- 
 spect from the interior. The term ' crust of the globe ' is 
 employed by geologists as a convenient way of referring 
 to that portion of the earth which is accessible to their 
 observation. 
 
 But if we are unable to make direct investigations 
 concerning the nature of the internal portions of the 
 globe, there are nevertheless a number of facts from 
 which we may draw important inferences upon the 
 subject. These facts and the inferences based upon 
 them we shall now proceed to consider. 
 
 First in importance among these we may mention 
 the results which have been obtained by weighing our 
 globe. Various methods have been devised for accom- 
 plishing this important object, and the conclusions 
 arrived at by different methods agree so closely with 
 one another, that there is no room for doubt as to the 
 substantial accuracy of those results. It may be taken 
 as proved beyond the possibility of controversy that 
 our globe is equal in weight to five and a half globes 
 of the same size composed of water, or, in other words, 
 that the average density of the materials composing the 
 globe is five and a half times as great as that of water. 
 
 Now the density of the materials which compose the 
 crust of the globe is very much less than this, varying 
 from about two-and-one-third to three times that of 
 water. Hence we are compelled to conclude that the 
 interior portions of the globe are of far greater density 
 than the exterior portions ; that, as a matter of fact. 
 
DENSITY OF EARTH'S INTERIOR. 309 
 
 the mass of the globe is composed of materials having 
 twice the density of the rocks exposed at the surface. 
 
 It has been sometimes argued that as all materials 
 under intense pressure appear to yield to an appreciable 
 extent, and to allow their particles to be packed into a 
 smaller compass, we may find in this fact an explanation 
 of the great density of the internal parts of the globe. 
 It has in fact been suggested that under the enormous 
 pressure which must be exerted by masses of rock 
 several thousand feet in thickness, the materials of 
 which our earth is composed may be compelled to pack 
 themselves into less than one-half the compass which 
 they occupy at the surface. But the ascription of such 
 almost unlimited compressibility to solid substances 
 can be supported neither by experiment nor analogy. 
 Various considerations point to the probability that 
 solid bodies yield to pressure up to a certain limit and S 
 no farther, and that when this limit is reached an in- 
 crease in pressure is no longer attended with a re- 
 duction in bulk. 
 
 If then we are compelled to reject the idea of the 
 unlimited compressibility of solid substances, we must 
 conclude that the interior portions of our globe are 
 composed of materials of a different kind from those 
 which occur in its crust. And this conclusion, as we 
 shall presently see, is borne out by a number of in- 
 dependent facts. 
 
 The study of the materials ejected from volcanic 
 vents proves that even at very moderate depths there 
 
310 VOLCANOES. 
 
 exist substances differing greatly in density, as well 
 as in chemical composition. The lightest lavas have a 
 specific gravity of 2-3, the heaviest of over 3. And that 
 materials of even greater density are sometimes brought 
 by volcanic action from the earth's interior, we have 
 now the clearest proofs. 
 
 But in considering a question of this kind, it will 
 be well to remember that analogy may furnish us with 
 hints upon the subject which may prove to be by no 
 means unimportant. There is no question upon which 
 modern science has wrought out a more complete re- 
 volution in our ideas, than that of the relation of our 
 earth to the other bodies of the universe. We know, 
 as the result of recent research, that our globe is one of 
 a great family of bodies, moving through space in 
 similar paths and in obedience to the same laws. A 
 hundred years ago the primary and secondary planets 
 of the solar system could be almost numbered upon 
 the fingers ; now we recognise the fact that they 
 exist in countless millions, presenting every variety 
 of bulk from masses 1,400 times as large as our 
 earth down to the merest planetary dust. Between the 
 orbits of Mars and Jupiter, more than 200 small planets 
 have been recognised as occurring, and every year 
 additions are made to the number of these asteroids. 
 Comets have now been identified with streams of such 
 planetary bodies, of minute size, moving in regular 
 orbits through our system. The magnificent showers 
 of ' shoot ing- stars ' have been proved to be caused by 
 
KELATION BETWEEN EAKTH AND OTHEK PLANETS. 311 
 
 the passage of the earth through such bands of travel- 
 ling bodies, and 'the zodiacal light' finds its most 
 probable explanation in the supposition that the sun 
 is surrounded by a great mass of such minute planets. 
 Every increase in the power of the telescope reveals to 
 us the existence of new secondary planets or moons, re- 
 volving about the primaries ; and the wonderful system 
 of the Saturnian rings is now explained by the proved 
 existence of great streams of such secondary planets 
 circling around it. The solar system was formerly con- 
 ceived of as a vast solitude through which a few gigantic 
 bodies moved at awful distances from one another. Now 
 we know that the supposed empty void is traversed by 
 countless myriads of bodies of the most varied dimen- 
 sions, all moving in certain definite paths, in obedience 
 to the same laws, ever acting and reacting upon each 
 other, and occasionally coming into collision. 
 
 There are not wanting further facts to prove that 
 the other planets are like our own in many of their 
 phenomena and surroundings. In some of them atmo- 
 spheric phenomena have been detected, such as the for- 
 mation of clouds and the deposition of snow, so that the 
 external forces at work on our globe act upon them 
 also. And that internal forces, like those we have been 
 considering in the case of our earth, are at work in our 
 neighbours, is proved by the great solar storms and the 
 condition of the moon's surface. 
 
 But the results of spectrum-analysis in recent years 
 have furnished new facts in proof of the close relation- 
 
314 VOLCANOES. 
 
 But this is by no means the case with the meteorites. 
 In them we find metals like iron, nickel, cobalt, &c., in 
 their uncombined condition, and forming alloys with 
 one another. The same and other metals also occur 
 in combination with carbon, phosphorus, chlorine, and 
 sulphur, and some of the substances thus formed are 
 quite unknown among terrestrial rocks. Compounds of 
 the oxide of silicon with the oxides of the metals such 
 as form the mass of the crust of the globe do occur in 
 meteorites, but they play a much less important part 
 than in the case of the terrestrial rocks. 
 
 Among the substances found in meteorites are 
 several which do not exist among the terrestrial rocks 
 some, indeed, which it seems impossible to conceive of 
 as being formed and preserved under terrestrial con- 
 ditions. Among these we may mention the phosphide 
 of iron and nickel (Schreibersite), the sulphide of 
 chromium and iron (Daubreelite), the protosulphide of 
 iron (Troilite), the sulphide of calcium (Oldhamite), 
 the protochloride of iron (Lawrencite), and a peculiar 
 form of crystallised silica, called by Professor Maskelyne 
 ' Asmanite.' 
 
 There are other phenomena exhibited by meteorites 
 which indicate that they must have been formed under 
 conditions very different to those which prevail upon 
 the earth's surface. Thus we find that fused iron and 
 molten slag-like materials have remained entangled with 
 each other, and have not separated as they would do 
 if a great body like the earth were near to exercise the 
 
DIFFERENT KINDS OF METEORITES. 315 
 
 varying force of gravity upon the two classes of sub- 
 stances. Again, meteorites are found to have absorbed 
 many times their bulk of hydrogen gas, and to exhibit 
 peculiarities in their microscopic structure which can 
 probably be only accounted for when we remember 
 that they were formed in the interplanetary spaces, far 
 away from any great attracting body. 
 
 But in recent years a number of very important 
 facts have been discovered which may well lead us to 
 devote a closer attention to the composition and struc- 
 ture of meteorites. It has been shown, on the one 
 hand, that some meteorites contain substances precisely 
 similar to those which are sometimes brought from the 
 earth's interior during volcanic outbursts ; and, on the 
 other hand, there have been detected, among some of 
 the ejections of volcanoes, bodies which so closely re- 
 semble meteorites that they were long mistaken for 
 them. Both kinds of observation seem to point to the 
 conclusion that the earth's interior is composed of 
 similar materials to those which we find in the small 
 planets called meteorites. 
 
 M. Daubree has proposed a very convenient classifi- 
 cation for meteorites, dividing them into the following 
 four groups : 
 
 I. Holosiderites ; consisting almost entirely of 
 metallic iron, or of iron alloyed with nickel, stony 
 matter being absent; but sulphides, phosphides, and 
 carbides of several metals are often diffused through 
 the mass. The polished surfaces of these meteoric 
 
318 VOLCANOES. 
 
 metals in the free or uncombined state. But not tm- 
 frequently we find among the ordinary ejections of 
 volcanoes, nodules and fragments of such ultra-basic 
 materials, which have clearly been carried up with the 
 other lavas from great depths in the earth's crust. 
 Thus in Auvergne, the Eifel, Bohemia, Styria, and 
 many other volcanic districts, the basaltic lavas and 
 tuffs are found to contain nodules composed of the 
 minerals which are so highly characteristic of meteo- 
 rites. Such nodules, too, often form the centres of 
 the volcanic bombs which are thrown out of craters 
 during eruptions. 
 
 We thus see that materials identical in composition 
 and character with the stony portions of meteorites, 
 exist within the earth's interior, and are thrown out on 
 its surface by volcanic action. A still more interesting 
 discovery has been made in recent years ; namely, that 
 materials similar to the metallic portion of meteorites, 
 and consisting of nickeliferous iron, also occur in deep- 
 seated portions of the earth's crust, and are brought to 
 the surface during periods of igneous activity. 
 
 In the year 1870, Professor Nordenskiold made a 
 most important discovery at Ovifak, on the south side 
 of the Island of Disko, off the Greenland coast. On 
 the shore of the island a number of blocks of iron were 
 seen, and the chemical examination of these proved 
 that, like ordinary metallic meteorites, they consisted 
 of iron alloyed with nickel and cobalt. 
 
 Now, when the facts concerning the masses of native 
 
IRON-MASSES OF OVIFAK. 
 
 310 
 
 iron of Ovifak were made known, the first and most 
 natural explanation which presented itself to every 
 mind was, that these were a number of meteorites 
 
 Metallic iron. 
 
 Opaque crystals of magnetite (black oxide of iron). 
 
 Transparent crystals of felspar, augite, and olivine. 
 FIG. 87. SECTION OF BASALT FROM OVIFAK, GREENLAND, WITH 
 
 PARTICLES OF METALLIC IRON DIFFUSED THROUGH ITS MASS. 
 
 which at some past period had fallen upon the earth's 
 surface. 
 
 But a further examination of the locality revealed 
 a number of facts which, as Professor Steenstrup 
 pointed out, it is very difficult to reconcile with the 
 theory that the Ovifak masses of iron are of meteoric 
 
320 VOLCANOES. 
 
 origin. The district of Western Greenland, where 
 these masses were discovered, has been the scene of 
 volcanic outbursts on the grandest scale during the 
 Miocene period. In close proximity to the great iron 
 masses, there are seen a number of basaltic dykes; 
 and, when these dykes are carefully examined, the 
 basaltic rock of which they are composed is seen to be 
 full of particles of metallic iron. In fig. 87, we have 
 a drawing made from a section of the Ovifak basalts 
 magnified four or five diameters. The rock-mass is 
 seen to be composed of black, opaque magnetite, and 
 transparent crystals of augite, labradorite, olivine, &c. ; 
 while, through the whole, particles of metallic iron are. 
 found entangled among the different crystals in the 
 most remarkable manner. 
 
 It has been suggested that this singular rock might 
 have been formed by a meteorite falling, in Miocene 
 times, into a lava-stream in a state of incandescence. 
 But the relation of the metallic particles to the stony 
 materials is such as to lend no support whatever to 
 this rather strained hypothesis. 
 
 A careful study of all the facts of the case by 
 Lawrence Smith, Daubree, and others well acquainted 
 with the phenomena exhibited by meteorites, has led to 
 the conclusion that the large iron-masses of Ovifak, a& 
 well as the particles of metallic iron diffused through 
 the surrounding basalts, are all of terrestrial origin, and 
 have been brought by volcanic action from the earth's 
 interior. It is probable that, just as we find in many 
 
MATERIALS FILLING METALLIC-VEINS. 821 
 
 basaltic lavas nodules of ultra-basic materials similar to 
 the stony parts of meteorites, so in these basalts of 
 Ovifak we have masses of iron alloyed with nickel, 
 similar to the metallic portions of meteorites. Both 
 the stony and metallic enclosures in the basalt are in 
 all probability derived from deeper portions of the 
 earth's crust. By the weathering away of the basalt 
 of Ovifak, the larger masses of metallic iron have been 
 left exposed upon the shore where they were found. 
 
 There are a number of other facts which seem to 
 support this startling conclusion. Thus it has been 
 shown by Professor Andrews that certain basalts in our 
 own islands contain particles of metallic iron of micro- 
 scopic dimensions, and it is not improbable that some 
 of the masses of nickeliferous iron found in various 
 parts of the earth's surface, which have hitherto been 
 regarded as meteorites, are, like those of Ovifak, of 
 terrestrial origin. 
 
 Another piece of evidence pointing in the same 
 direction, is derived from those great fissures communi- 
 cating with the interior of our globe which become filled 
 with metallic minerals, and are known to us as mineral- 
 veins. In these mineral-veins the native metals, their 
 alloys, and combinations of these with sulphur, chlorine, 
 phosphorus, &c., are frequently present. But oxides of 
 the metals, except as products of subsequent alteration, 
 occur far less frequently than in the earth's crust gene- 
 rally. Hence we are led to conclude that the substances 
 which in the outer part of the earth's crust always exist 
 15 
 
322 VOLCANOES. 
 
 in combination with oxygen, are at greater depths in a 
 free and uncombined condition. 
 
 Nor is it a circumstance altogether unworthy of 
 attention that the researches of Mr. Norman Lockyer 
 and other astronomers, based on the known facts of the 
 relative densities of the several members of the solar 
 system, and the ascertained relations of the different 
 solar envelopes, have led to conclusions closely in accord 
 with those arrived at by geologists. These researches 
 appear to warrant the hypothesis that the interior of 
 our globe consists of metallic substances uncombined 
 with oxygen, and that among these metallic substances 
 iron plays an important part. Our globe, as we know, 
 is a great magnet, and the remarkable phenomena of 
 terrestrial magnetism may also not improbably find 
 their explanation in the fact that metallic iron forms 
 so large a portion of the earth's interior. 
 
 The interesting facts which we have been consider- 
 ing may be made clearer by the accompanying diagram 
 (fig. 88). The materials ejected from volcanic vents 
 (lavas) are in almost all cases compounds of silicon and 
 the various metals with oxygen. In the lighter or acid 
 lavas oxygen constitutes one-half of their weight, and 
 the proportion of metals of the iron-group is very small. 
 As we pass to the heavier intermediate and basic lavas, 
 we find the proportion of oxygen diminishing, and the 
 metals of the alkaline earths (magnesium and calcium) 
 with the metals of the iron-group increasing, in quan- 
 tity. In the small and interesting group of the ultra- 
 
TERRESTRIAL AND EXTRA-TERRESTRIAL ROCKS. 323 
 
 basic lavas the proportion of oxygen is comparatively 
 small, and the proportion of magnesium and iron very 
 high. So much for the terrestrial rocks. 
 
 Now let us turn our attention to the extra-terrestrial 
 rocks or those found in meteorites. The Asiderites are 
 quite identical in composition with the ultra-basic lavas 
 of our globe, but in the Sporadosiderites and the Sys- 
 siderites we find the proportion of oxygen rapidly dimi- 
 nishing, and that of metallic iron increasing. Finally, 
 in the Holosiderites the oxygen entirely disappears, 
 and the whole mass becomes metallic. 
 
 From the Holosiderites at one end of the chain to 
 the acid lavas at the other, we find there is a complete 
 and continuous series; the rocks of terrestrial origin 
 overlapping, in their least oxydized representatives, the 
 most highly oxydized representatives of the extra-ter- 
 restrial rocks. But the discovery at Ovifak of the iron- 
 masses, and the basalts with iron disseminated, has 
 afforded another very important link, placing the ter- 
 restrial and extra-terrestrial rocks in closer relations 
 with one another. 
 
 All these facts appear to point to the conclusion 
 that the earth's interior consists of metallic substances 
 either quite uncombined or simply alloyed with one 
 another, and among these iron is very conspicuous by 
 its abundance. The outer crust, which is probably of 
 no great thickness, contains an enormous proportion of 
 oxygen and silicon combined with the materials which 
 constitute the interior portions of our globe. It may 
 
324 VOLCANOES. 
 
 be, as has been suggested by astronomers, that our 
 earth consisted at one time of a solid metallic mass 
 surrounded by a vaporous envelope of metalloids, and 
 that the whole of the latter, with the exception of the 
 constituents of the atmosphere and ocean, have gradu- 
 ally entered into combination with the metals of the 
 nucleus to form the existing crust of the globe. But 
 of this period the geologist can take no cognisance. 
 The records which he studies evidently commenced at 
 a long subsequent period, when the conditions prevail- 
 ing at the earth's surface differed but little, if at all, 
 from those which exist at the present day. Equally little 
 has the geologist to do with speculations concerning 
 a far distant future when, as some philosophers have 
 suggested, the work of combination of the waters and 
 atmosphere of the earth's surface with the metallic 
 substances of its interior shall be completed, and our 
 globe, entirely deprived of its fluid envelopes, reduced to 
 the condition in which we find our satellite, the moon. 
 
 There is another class of enquiries concerning the 
 earth's interior to which the attention of both geologists 
 and astronomers has long been directed that, namely, 
 which deals with the problem of the physical condition 
 of the interior of our globe. 
 
 The fact that masses of molten materials are seen 
 at many points of the earth's surface to issue from 
 fissures in the crust of our globe, seems at first sight 
 to find a simple explanation if we suppose our planet to 
 
PHYSICAL CONDITION OF EARTH'S INTERIOR. 325 
 
 consist of a fluid central mass surrounded by a solid 
 crust. Hence we find that among those who first thought 
 upon this subject, this hypothesis of a liquid centre 
 and a solid crust was almost universally accepted. This 
 hypothesis was supposed to find further support in 
 the fact that, as we penetrate into the earth's crust by 
 mines or boring operations, the temperature is found 
 to continually increase. It was imagined, too, that 
 this condition of our planet would best agree with the 
 requirements of the nebular hypothesis of Laplace, 
 which explains the formations and movements of the 
 bodies of the solar system by the cooling down of 
 a nebulous mass. 
 
 But a more careful and critical examination of the 
 question has led many geologists and astronomers to 
 reject the hypothesis that the earth consists of a great 
 fluid mass surrounded by a comparatively thin shell 
 of solid materials. 
 
 Volcanic outbursts and earthquake tremors, though 
 so terrible and destructive to man and his works, are 
 but slight and inconsiderable disturbances in a globe of 
 such vast dimensions as that on which we live. The 
 condition of the crust of the globe is, in spite of volcanic 
 and earthquake manifestations, one of general stability ; 
 and this general stability has certainly been main- 
 tained during the vast periods covered by the geological 
 record. Such a state of things seems quite irrecon- 
 cilable with the supposition that, at no great depth 
 from the surface, the whole mass of the globe is in a 
 
326 VOLCANOES. 
 
 liquid condition. If, on the other hand, it be supposed 
 that the solid crust of the globe is several hundreds of 
 miles in thickness, it is difficult to understand how the 
 local centres of volcanic activity could be supplied 
 from such deep-seated sources. 
 
 There are other facts which seem equally irreconcil- 
 able with the hypothesis of a fluid centre and a thin 
 solid crust in our globe. If all igneous products were 
 derived from one central reservoir, we might fairly 
 expect to find a much greater uniformity of character 
 among those products than really exists. But in some 
 cases, materials of totally different composition are 
 ejected at the same time from closely adjoining vol- 
 canic districts. Thus in Hungary and Bohemia, as we 
 have seen, lavas of totally different character were being 
 extruded during the Miocene period. In the island of 
 Hawaii, as Professor Dana has pointed out, igneous 
 ejections have taken place at a crater 14,000 feet above 
 the sea-level, while a closely adjoining open vent at a 
 level 10,000 feet lower exhibited no kind of sympathy 
 with the disturbance. Whatever may be the cause of 
 volcanic action, it seems clear that it does not originate 
 in a universal mass of liquefied material situated at no 
 great depth from the earth's surface. 
 
 The conclusions arrived at by astronomers and 
 physicists is one quite in accord with those which 
 geologists have reached by totally different methods. 
 It is now very generally admitted that if the earth 
 were not a rigid mass, its behaviour under the attrac- 
 
ARGUMENTS AGAINST LIQUID INTERIOR. 327 
 
 tive influences of the surrounding members of the solar 
 system would be very different to what is found to be 
 the case. 
 
 That the earth is in a solid condition to a great 
 depth from the surface, and possibly quite to the centre, 
 is a conclusion concerning which there can be little 
 doubt ; and in the next chapter we shall endeavour to 
 show that such a condition of things is by no means 
 incompatible with those manifestations of internal 
 energy, the phenomena of which we are considering in 
 this work. The question, therefore, of the complete 
 solidity of our globe, or of its consisting of a solid and 
 a liquid portion, is one of speculative interest only, and 
 is in no way involved in our investigations concern- 
 ing the nature and origin of volcanic activity. We 
 may conclude this chapter by enumerating the several 
 hypotheses which have at different times been main- 
 tained concerning the nature of the interior of our globe. 
 
 First. It has been suggested that the earth con- 
 sists of a fluid or semi-fluid nucleus surrounded and 
 enclosed in a solid shell. Some have maintained this 
 shell to be of such insignificant thickness, as compared 
 with the bulk of the interior liquid mass, that portions 
 of the latter are able to reach the earth's surface through 
 movements and fractures of the outer shell, and that in 
 this manner volcanic manifestations originate. Others, 
 impressed with the general stability and rigidity of the 
 globe as a whole, have maintained that the outer solid 
 shell must have a very considerable thickness, amount- 
 
328 VOLCANOES. 
 
 ing probably to not less than several hundreds of miles. 
 But through a shell of such thickness it is difficult to 
 conceive of the liquid masses of the interior finding 
 their way to the surface, and those who have held this 
 view are driven to suggest some other means by which 
 local developments of volcanic action might be brought 
 about. 
 
 Secondly. Some physicists have asserted that a globe 
 of liquid matter radiating its heat into space, would tend 
 to solidify both at the surface and the centre, at the same 
 time. The consequence of this action would be the 
 production of a sphere with a solid external shell and 
 a solid central nucleus, but with an interposed layer in 
 a fluid or semi-fluid condition. It has been pointed 
 out that if we suppose the solidification to have gone 
 so far, as to have caused the partial union of the in- 
 terior nucleus and the external shell, we may conceive 
 a condition of things in which the stability and 
 rigidity is sufficient to satisfy both geologists and as- 
 tronomers, but that in still unsolidified pockets or 
 reservoirs, filled with liquefied rock, between the nu- 
 cleus and the shell, we should have a competent cause 
 for the production of the volcanic phenomena of the 
 globe. In this hypothesis, however, it is assumed 
 that the cooling at the centre and the surface of the 
 globe would go on at such rates that the reservoirs ot 
 liquid material would be left at a moderate depth from 
 the surface, so that easy communication could be 
 opened between them and volcanic vents. 
 
EEVIEW OF THE SEVERAL HYPOTHESES. 329 
 
 Thirdly. It has been maintained that the earth 
 may have become perfectly solid from the centre to 
 the surface. Those who hold this view endeavour to 
 account for the phenomena of volcanoes in one of two 
 ways. It may be, they say, that the deep-seated rock- 
 masses, though actually solid, are in a state of poten- 
 tial liquidity; that though reduced to a solid state by 
 the intense pressure of the superincumbent masses, yet 
 such is the condition of unstable equilibrium in the 
 whole mass, that the comparatively slight movements 
 and changes taking place at the earth's surface suffice 
 to bring about the liquefaction of portions of its crust 
 and consequent manifestations of volcanic energy. But 
 it may be, as other supporters of the doctrine of the 
 earth's complete solidity have maintained, that the 
 phenomena of volcanoes have no direct connection with 
 a supposed incandescent condition of our planet at all, 
 and that there are chemical and mechanical forces at 
 work within our globe which are quite competent to 
 produce at the surface all those remarkable phenomena 
 which we identify with volcanic action. 
 
 From this summary of the speculative views which 
 have been entertained upon the subject of the physical 
 condition of the earth's interior, it will be clear that at 
 present we have not sufficient evidence for arriving at 
 anything like a definite solution of the problem. The 
 conditions of temperature and pressure which exist in 
 the interior of a globe of such vast dimensions as our 
 earth, are so far removed from those which we can 
 
330 VOLCANOES. 
 
 imitate in our experimental enquiries, and it is so un- 
 safe to push the application of laws arrived at by the 
 latter to the extreme limits required by the former, 
 that we shall do well to pause before attempting to 
 dogmatise on such a difficult question. 
 
 In the next chapter we shall endeavour to grapple 
 with a somewhat more hopeful task, to point out how 
 far observation and experiment have enabled us to 
 offer a reasonable explanation of the wonderful series 
 of phenomena which are displayed during outbursts of 
 volcanic activity. 
 
CHAPTER XII. 
 
 THE ATTEMPTS WHICH HAVE BEEN MADE TO EXPLAIN 
 THE CAUSES OF VOLCANIC ACTION. 
 
 EVERY completed scientific investigation must consist 
 of four series of operations. In the first of these an 
 attempt is made to collect the whole of the facts 
 bearing on the question, by means of observation and 
 experiment; the latter being only observation under 
 conditions determined by ourselves. In the second 
 stage of the enquiry, the attention is directed to classi- 
 fying and grouping the isolated facts, so as to deter- 
 mine their bearings upon one another, and the general 
 conclusions to which they appear to point. In the 
 third stage, it is sought to frame an hypothesis which 
 shall embrace all the observed facts, and shall be in 
 harmony with the general conclusions derived from 
 them. In the fourth stage, this hypothesis is put to 
 the most rigid test ; comparing the results which must 
 follow, if it be true, with the phenomena actually ob- 
 served, and rejecting or amending our hypothesis accord- 
 ingly. Every great scientific theory has thus been 
 
332 ...... VOLCANOES. 
 
 established by these four processes observation, gene- 
 ralisation, hypothesis, and verification. 
 
 The enquiry concerning the nature and causes of 
 volcanic action is far from being a completed one. It 
 is true that many hypotheses upon the subject have 
 been framed, but in too many instances these have not 
 been based on accurate observations and careful gene- 
 ralisations, and can be regarded as little better than 
 mere guesses. Indeed, the state of the enquiry at the 
 present time would seem to be as follows. Although 
 much remains to be done in the direction both of 
 observation and experiment, the main facts of the case 
 have been established upon irrefragable evidence. The 
 classification and comparison of these facts have led to 
 the recognition of certain laws, which seem to embrace 
 all the known facts. To account for these facts and 
 their demonstrated relations to one another, certain 
 tentative hypotheses have been suggested ; but in no 
 case can it be truly said that these latter have so far 
 stood the test of exact enquiry as to deserve to rank 
 as demonstrated truths. A complete and consistent 
 theory of volcanic action still remains to be discovered. 
 
 In accordance with the plan which we have sketched 
 out for ourselves at the commencement of this work, 
 we shall aim at following what has been the order of 
 investigation and discovery in our study of volcanic 
 action ; and in this concluding chapter we shall indi- 
 cate the different hypotheses by which it has been 
 proposed to account for the varied phenomena, which 
 
VALUE AND LIMITS OF 
 
 we have discussed in the preceding pages, and their 
 remarkable relations to one another. We shall en 
 deavour, in passing, to indicate how far these several 
 hypotheses appear to be probable, as satisfying a larger 
 or smaller number of those conditions of the problem 
 which have been established by observation, experi- 
 ment, and careful reasoning ; but we shall at the same 
 time carefully avoid such advocacy of any particular 
 views as would tend to a prejudgment of the question. 
 Hypothesis is, as we have seen, one of the legitimate 
 and necessary operations in scientific investigation. It 
 only becomes a dangerous and treacherous weapon 
 when it is made to precede rather than to follow obser- 
 vation and experiment, or when being regarded with 
 paternal indulgence, an attempt is made to shield 
 it from the relentless logic of facts. Good and bad 
 hypotheses must be allowed to ' grow together till the 
 harvest ; ' such as are unable to accommodate them- 
 selves to the surrounding conditions imposed by 
 newly-discovered facts and freshly-established laws will 
 assuredly perish ; and in this c struggle for existence ' 
 the true hypothesis will in the end survive, while the 
 false ones perish. 
 
 It may well happen, however, that among the 
 hypotheses which have up to the present time been 
 framed, none will be found to entirely satisfy all the 
 conditions of the problem. New discoveries in physics 
 and chemistry have suggested fresh explanations of 
 volcanic phenomena in the past, and may continue to 
 
334 VOLCANOES. 
 
 do so in the future; and the true theory of volcanic 
 action, when it is at last discovered, may combine 
 many of the principles which now seem to be peculiar 
 to different hypotheses. 
 
 Let us, in the first place, enquire what are the facts 
 which must be accounted for in any theory of volcanic 
 action. We have already been led to the conclusion 
 that the phenomena exhibited by volcanoes were en- 
 tirely produced by the escape of imprisoned water and 
 other gases from masses of incandescent and fluid rock. 
 Our subsequent examination of the problem confirmed 
 the conclusion that in all cases of volcanic outburst we 
 have molten rock-materials from which water and other 
 gases issue with greater or less violence. The two 
 great facts to be accounted for, then, in any attempted 
 explanation of volcanic phenomena, are the existence 
 of this high temperature at certain points within the 
 earth's crust, and the presence of great quantities of 
 water and gas, imprisoned in the rocks. We shall 
 perhaps simplify the enquiry if we examine these two 
 questions separately, and, in the first place, review those 
 hypotheses which have been suggested to account for 
 high temperatures in the subterranean regions, and, in 
 the second place, examine those which seek to explain 
 the presence of large quantities of imprisoned water 
 and gases. 
 
 That a high temperature exists in the earth's crust 
 at some depth from the surface is a fact which does not 
 admit of any doubt. Every shaft sunk for mining 
 
INCREASE OF TEMPERATURE WITH DEPTH. 335 
 
 operations, and every deep boring made for the purpose 
 of obtaining water, proves that a more or less regular 
 increase of temperature takes place as we penetrate 
 downwards. The average rate of this increase of tem- 
 perature has been estimated to be about 1 Fahrenheit 
 for every 50 or 60 feet of depth. 
 
 Now if it be assumed that this regular increase of 
 temperature continues to great depths, a simple calcu- 
 lation proves that at a depth of 9,000 feet a temperature 
 of 212 Fahrenheit will be found one sufficient to boil 
 water at the earth's surface while at a depth of 28 
 miles the temperature will be high enough to melt 
 cast-iron, and at 34 miles to fuse platinum. 
 
 So marked is this steady increase of temperature as 
 we go downwards, that it has been seriously proposed 
 to make very deep borings in order to obtain supplies 
 of warm water for heating our towns. Arago and Wal- 
 ferdin suggested this method for warming the Jardin 
 des Plantes at Paris ; and now that such important 
 improvements have been devised in carrying borings to 
 enormous depths, the time may not be far distant when 
 we shall draw extensively upon these supplies of sub- 
 terranean heat. At the present time the city of Buda- 
 Pesth is extensively supplied with hot-water from an 
 underground source. Should our coal-supply ever fail 
 it may be well to remember that we have these inex- 
 haustible supplies of heat everywhere beneath our 
 feet. 
 
 But although we may conclude that at the moderate 
 
336 VOLCANOES. 
 
 depths we have indicated such high temperatures exist, 
 it would not be safe to infer, as some have done, that 
 at a distance of only 40 or 50 miles from the surface 
 the materials composing our globe are in a state of 
 actual fusion. Both theory and experiment indicate 
 that under increased pressure the fusing point of solid 
 bodies is raised ; and just as in a Papin's digester we 
 may have water retained by high pressure in a liquid 
 condition at a temperature far above 212 F., so in the 
 interior of the earth, masses of rock may exist in a 
 solid state, at a temperature far above that at which 
 they would fuse at the earth's surface. We may speak 
 of such rock-masses, retained in a solid condition by 
 intense pressure, at a temperature far above their fusing 
 point at the earth's surface, as being in a fi potentially 
 liquid condition.' Upon any relief of pressure such 
 masses would at once assume the liquid state, just 
 as the superheated water in a Papin's digester imme- 
 diately flashes into steam upon the fracture of the 
 strong vessel by which it is confined. We have already 
 seen how the action at volcanic vents often appears to 
 indicate just such a manifestation of elastic forces, as 
 would be exhibited by the relief of superheated masses 
 from a state of confinement by pressure. 
 
 In reasoning upon questions of this kind, however, 
 we must always be upon our guard against giving un- 
 due extension to principles and laws which seem to be 
 clearly established by experiment at the earth's surface. 
 It is well to remember how exceedingly limited is our 
 
 
EFFECT OF PEESSUKE ON FUSION-POINT. 337 
 
 command of extreme pressures and high temperatures, 
 when compared with those which may exist within a 
 body of the dimensions of our globe. 
 
 If we were to imagine a set of intelligent creatures, 
 who were able to command only a range of temperatures 
 from 50 to 200 F., engaged upon an investigation of 
 the properties of water, we shall easily understand how 
 unsafe it may be to extend generalisations far beyond 
 the limits covered by actual experiment. Such beings, 
 from their observation of the regular changes of volume 
 of water at all the temperatures they could command, 
 might infer that at still higher and lower temperatures 
 the same rates of expansion and contraction would be 
 maintained. Yet, as we well know, such an inference 
 would be quite wide of the truth ; for a little above 
 200 F. water suddenly expands to 1,700 times its 
 volume, and not far below 50 F. the contraction is 
 suddenly changed for expansion. 
 
 It has been argued by the late Mr. David Forbes 
 and others that, inasmuch as experiment has shown 
 that though the fusing points of solids are raised by 
 pressure, yet that this rise of the fusing points goes 
 on in a diminishing ratio as compared with the 
 pressures applied a limit will probably be reached at 
 which the most intense pressure will not be sufficient 
 to retain substances at a high temperature in their 
 solid state. The fact that gases cannot be retained in 
 a liquid condition by the most intense pressure at a 
 temperature above their critical point, may seem by 
 
338 VOLCANOES. 
 
 analogy to favour the same conclusion. Hence, David 
 Forbes, Dana, and other authors, have argued in favour 
 of the existence of a great liquid nucleus in our globe 
 covered by a comparatively thin, solid crust. And if 
 we accept the supposed proofs of a constant increase of 
 temperature from the surface to the centre of the globe, 
 such a conclusion appears to be at least as well founded 
 as that which regards the central masses of the earth 
 as maintained in a solid condition by intense pressure. 
 
 A little consideration will, however, convince us 
 that the facts which have been relied upon as proving 
 the intensely heated condition of the central masses of 
 our globe, are by no means so conclusive as has been 
 supposed. 
 
 The earth's form, which mathematicians have shown 
 to be exactly that which would be acquired by a globe 
 composed of yielding materials rotating on its axis at 
 the rate which our planet does, has often been adduced 
 as proving that the latter was not always in a rigid and 
 unyielding condition. In the same way, all the re- 
 markable facts and relations of the bodies of the solar 
 system, which have been shown by astronomers to lend 
 such support to the nebular hypothesis, have been 
 thought, at the same time, to favour the view that our 
 earth is still in a condition of uncompleted solidification. 
 
 But it is quite admissible to accept the nebular 
 hypothesis and the view that our globe attained its 
 present form while still in a state of fluidity, and at the 
 same time to maintain that our earth has long since 
 
SUPPOSED PKOOFS OF LIQUID NUCLEUS. 339 
 
 reached its condition of complete solidification. And 
 there are not a few facts which appear to lend support 
 to such a conclusion. 
 
 If the rapid rate of increase in temperature which 
 has been demonstrated to occur at so many parts of 
 the earth's surface be maintained to the centre, then, as 
 argued by David Forbes and Dana, it is difficult to 
 conceive of our earth as being in any other condition 
 than that of a liquid mass covered by a comparatively 
 thin crust. The objection to this view, both upon geo- 
 logical and astronomical grounds, we have pointed out 
 in the previous chapter. 
 
 Before accepting as a demonstrated conclusion this 
 notion of a constant increase of temperature from the 
 surface to the centre of our globe, it may be well to 
 re-examine the facts which are relied upon as proving it. 
 
 That there is a general increase of temperature so 
 far as we are able to go downwards in the earth's crust, 
 there can, as we have seen, be no doubt whatever. Yet 
 it may be well to bear in mind how very limited is the 
 range of our observation on the subject. The deepest 
 mines extend to little more than half-a-mile from the 
 surface, and the deepest borings to little more than 
 three-quarters of a mile, while the distance from the 
 earth's surface to its centre is nearly 4,000 miles. 
 We may well pause before we extend conclusions, de- 
 rived from such very limited observations, to such 
 enormous depths. 
 
 But when we examine critically these observations 
 
340 VOLCANOES. 
 
 themselves, we shall find equal grounds for caution in 
 generalising from them. There is the greatest and 
 most startling divergence in the results of the observa- 
 tions which have been made at different points at the 
 earth's surface. Even when every allowance is made 
 for errors of observation, these discrepancies still remain. 
 In some places the increase of temperature as we go 
 downwards is so rapid that it amounts to 1 Fahrenheit 
 for every 20 feet in depth, while in other cases, in order 
 to obtain the same increase in temperature of 1 Fah- 
 renheit, we have to descend as much as 100 feet. 
 
 JSTow if, as is so often assumed, this increase of 
 temperature as we go downwards be due to our approach 
 to incandescent masses forming the interior portions of 
 the globe, it is difficult to understand why greater 
 uniformity is not exhibited in the rate of increase in 
 different areas. No difference in the conducting powers 
 of the various rock-materials is sufficient to account for 
 the fact that in some places the rate of increase in 
 temperature in going downwards is no less than five 
 times as great as it is in others. 
 
 Again, there are some remarkable facts concerning 
 the variation in the rate of increase in temperature 
 with depth which seem equally irreconcilable with the 
 theory that the heat in question is directly derived from 
 a great, central, incandescent mass. M. Walferdin,by a 
 series of careful observations in two shafts at Creuzot, 
 proved that down to the depth of 1,800 feet the increase 
 of temperature amounted to 1 Fahrenheit for every 55 
 
VAKIATIONS IN UNDERGROUND TEMPERATUEES. 341 
 
 feet of descent, but below the depth named, the rate of 
 increase was as much as 1 Fahrenheit for every 44 feet. 
 On the other hand, in the great boring of Grenelle at 
 Paris, the increase in temperature down to the depth of 
 740 feet amounted to 1 Fahrenheit for every 50 feet of 
 descent, but from 740 feet down to 1,600 feet, the rate 
 of increase diminished to 1 for 75 feet of descent. 
 The same remarkable fact was strikingly shown in the 
 case of the deepest boring in the world that of Speren- 
 berg, near Berlin, which attained the great depth of 
 4,052 feet. In this case, the rate of increase in tem- 
 perature for the first 1,900 feet, was 1 Fahrenheit 
 for every 55 feet of descent, and for the next 2,000, it 
 diminished to 1 Fahrenheit for every 62 feet of descent. 
 In the deep well of Buda-Pesth there was actually found 
 a decline in temperature below the depth of 3,000 
 feet. 
 
 Perhaps the most interesting fact in connection 
 with this question which has been discovered of late 
 years, is that in districts which have recently been the 
 seat of volcanic agencies, the rate of increase in tem- 
 perature, as we go downwards in the earth's crust, is 
 abnormally high. Thus at Monte Massi in Tuscany, 
 the temperature was found to increase at the rate 
 of 1 Fahrenheit for every 24 feet of descent. In 
 Hungary several deep wells and borings have been 
 made, which prove that a very rapid increase of tem- 
 perature occurs. The deep boring at Buda-Pesth pene- 
 trates to a depth of 3,160 feet, and a temperature of 
 
342 VOLCANOES. 
 
 178 Fahrenheit has been observed near the bottom. 
 The rate of increase of temperature in this boring was 
 about 1 for every 23 feet of descent. In the mines 
 opened in the great Comstock lode, in the western 
 territories of the United States, an abnormally high 
 temperature has been met with amounting in some 
 cases to 157 Fahrenheit. Although this is the richest 
 mineral- vein in the world, having yielded since 1859, 
 when it was first discovered, 60,000,000^. worth of gold 
 and silver, this rapid increase in temperature in going 
 downwards threatens in the end to entirely baffle the 
 enterprise of the miner. The rate of increase in tem- 
 perature in the case of the Comstock mines has been 
 estimated at 1 Fahrenheit for every 45 feet of descent, 
 between 1,000 and 2,000 feet from the surface, but as 
 much as 1 Fahrenheit for every 25 feet, at depths 
 below 2,000 feet. 
 
 The facts which we have stated, with others of a 
 similar kind, have led geologists to look with grave feel- 
 ings of doubt upon the old hypothesis which regarded 
 the increase of temperature found in making excavations 
 into the earth's crust as a proof that we are approach- 
 ing a great incandescent nucleus. They have thus been 
 led to enquire whether there are any conceivable sources 
 of high temperatures at moderate depths temperatures 
 which would be quite competent to produce locally all 
 the phenomena of volcanic action. 
 
 There are not wanting other facts which seem to 
 point to the same conclusion : namely, that volcanic 
 
DEPTHS AT WHICH EARTHQUAKES ORIGINATE. 343 
 
 action is not due to the existence of a universal reser- 
 voir of incandescent material occupying the central 
 portion of our globe, but to the local development 
 of high temperatures at moderate depths from the 
 surface. 
 
 The close connection between the phenomena of vol- 
 canoes and earthquakes cannot be doubted. It is true 
 that some of those vibrations or tremors of the earth's 
 crust, to which we apply the name of earthquakes, occur 
 in areas which are not now the seat of volcanic action ; 
 and it is equally true that the stratified rock-masses of 
 our globe, far away from any volcanic centres, exhibit 
 proofs of violent movement and fracture, in the pro- 
 duction of wilich, concussions giving rise to earthquake 
 vibrations, could scarcely fail to have occurred. But it 
 is none the less certain that earthquakes as a rule take 
 place in those areas which are the seats of volcanic action, 
 and that great earthquake-shocks precede and accom- 
 pany volcanic outbursts. Sometimes, too, it has been 
 noticed that the manifestation of activity at a volcanic 
 centre is marked by the sudden decline of the earth- 
 quake-tremors of the district around, as though a 
 safety-valve had been opened at that part of the earth's 
 surface. 
 
 Mr. Mallet has shown that by the careful study of 
 the effects produced at the surface by earthquake-vibra- 
 tions, we may determine with considerable accuracy the 
 point at which the shock or concussion occurred which 
 gave rise to the vibration. Now it is a most remarkable 
 
344 VOLCANOES. 
 
 fact that such calculations have led to the conclusion 
 that, so far as is at present known, earthquake shocks 
 never originate at greater depths than thirty miles from 
 the surface, and that in some cases the focus from which 
 the waves of elastic compression producing an earth- 
 quake proceed is only at the depth of seven or eight 
 miles. As we have already seen, there can be no doubt 
 that in the great majority of instances the forces 
 originating earthquake-vibrations and volcanic outbursts 
 are the same, and independent lines of reasoning have 
 conducted us to the conclusion that these forces operate 
 at very moderate distances from the earth's surface. 
 
 Under these circumstances, geologists have been led 
 to enquire whether there are any means by which we 
 can conceive of such an amount of heat, as would be 
 competent to produce volcanic outbursts, being locally 
 developed at certain points within the earth's crust. 
 Recent discoveries in physical science which have shown 
 the close relation to one another of different kinds of 
 force, and their mutual convertibility, have at least 
 suggested the possibility of the existence of causes by 
 which such high temperatures within certain portions 
 of the earth's crust may be originated. 
 
 When, at the commencement of the present century, 
 Sir Humphry Davy discovered the remarkable metals 
 of the alkalies and alkaline earths, and at the same time 
 demonstrated the striking phenomena which are ex- 
 hibited if these metals be permitted to unite with 
 oxygen, he at once perceived that if such metals existed 
 
DAVY'S CHEMICAL THEOKY. 345 
 
 in an uncombined condition within the earth's crust, 
 the access of water and air to the mass might give rise 
 to the development of such an amount of heat, as would 
 be competent to produce volcanic phenomena at the 
 surface. It is true that at a later date Davy recognised 
 the chemical theory of volcanoes as being beset with 
 considerable difficulties, and was disposed to abandon 
 it altogether. It was argued, with considerable show 
 of reason, that if the heat at volcanic centres were 
 produced by the access of water to metallic substances, 
 great quantities of hydrogen would necessarily be 
 evolved, and this gas ought to be found in prodigious 
 quantities among the emanations of volcanoes. The 
 fact that such enormous quantities of hydrogen gas are 
 not emitted from volcanic vents has been held by many 
 authors to be fatal to the chemical theory of volcanoes. 
 
 But the later researches of Graham and others have 
 made known facts which go far towards supplying an 
 answer to the objections raised against the chemical 
 theory of volcanoes. Various solids and liquids have 
 been shown to possess the power of absorbing many 
 times their volume of certain gases. Among the gases 
 thus absorbed in large quantities by solids and liquids, 
 hydrogen is very conspicuous. In some cases gases are 
 absorbed by metals or other solids in a state of fusion, 
 and yielded up again by them as they cool. 
 
 It is a very remarkable circumstance that some 
 meteorites are found to have absorbed large quantities 
 of hydrogen gas, and this is given off when they are 
 16 
 
346 VOLCANOES. 
 
 heated in vacuo. Thus it has been demonstrated that 
 certain meteorites have contained as much as forty- 
 seven times their own volume of hydrogen gas. 
 
 We have already pointed out that there are reasons 
 for believing the internal portions of our globe to be 
 composed of materials similar to those found in meteo- 
 rites. If such be the case, the access of water to these 
 metallic substances may result in the formation of 
 oxides, attended with a great local development of heat, 
 the hydrogen which is liberated being at once absorbed 
 by the surrounding metallic substances. That this 
 oxidation of the metallic substances in the interior 
 of our globe by the access of water and air from the 
 surface is continually going on, can scarcely be doubted. 
 We may even look forward to a far-distant period when 
 the whole of the liquid and gaseous envelopes of the 
 globe shall have been absorbed into its substance, and 
 our earth thereby reduced to the condition in which 
 we now find the moon to be. 
 
 There is a second method by which high tempera- 
 tures might be locally developed within the earth's 
 crust, which has been suggested by Vose, Mallet, and 
 other authors. 
 
 We have good grounds for believing that the tem- 
 perature of our globe is continually diminishing by its 
 radiation of heat into space. This cooling of our globe 
 is attended by contraction, which results in movements 
 of portions of its crust. It may at first sight appear 
 that such movements would be so small and insigm'fi- 
 
DYNAMICAL THEOEIES. 347 
 
 cant as to be quite unworthy of notice. But if we 
 take into account the vast size of our earth it will be 
 seen that the movements of such enormous masses 
 may be attended with the most wonderful results. 
 
 It has been shown that if a part of the earth's crust 
 fifty miles in thickness were to have its temperature 
 raised 200 Fahrenheit, its surface would be raised to 
 the extent of 1,000 or 1,500 feet, Le Conte has 
 pointed out that if we conceive the conduction of heat 
 to take place at slightly different rates along different 
 radii of our globe, we should at once be able to account 
 for the existing inequalities of the earth's surface, and 
 for all those continental movements which can be shown 
 to have taken place in past geological periods. 
 
 But if we admit, as we have good grounds for doir,g, 
 that the loss of heat from the external portions of our 
 globe goes on more rapidly than in the case of the ; 
 central masses, we have thereby introduced another 
 powerful agent for the production of high temperatures 
 within the earth's crust. The external shell of the< 
 globe will tend to contract upon the central mass, and 
 in so doing a series of tangential strains will result ; ' 
 which will be capable of folding and crumpling the 
 rocks along any lines of weakness. That such crush- 
 ing and crumpling has during all geological periods 
 taken place along lines of weakness in the earth's crust, 
 is proved, as we have seen, by the phenomena pre- 
 sented by mountain-ranges. Now these crushings, 
 crumplings, and other violent movements of great 
 
348 VOLCANOES. 
 
 rock-masses must result in the development of a vast 
 amount of heat, just as the forcing down of a break 
 upon a moving wheel produces heat. This conclusion 
 is strikingly confirmed by the well-known geological 
 fact that nearly all rocks which have undergone great 
 movement and contortion are found to present evidence 
 of having been subjected to such chemical and crystal- 
 line actions, as would result from the development of a 
 high temperature within their mass. 
 
 Let us sum up briefly the various methods which 
 have been suggested to account for the high tempera- 
 tures within certain parts of the earth's crust by which 
 volcanic phenomena are produced. 
 
 Our globe may be conceived of as an incandescent 
 liquid mass surrounded by a cooler, solid shell. If we 
 regard this liquid interior mass as supplying directly 
 the various volcanic vents of the earth, it must be con- 
 ceded that the outer shell is of comparatively slight 
 thickness. But astronomers are almost universally 
 agreed that such a thin outer shell and inner liquid 
 mass are quite incompatible with that rigidity which 
 our planet exhibits under the attractions of its neigh- 
 bours. Geologists are almost equally unanimous in 
 regarding this hypothesis of a liquid nucleus and thin, 
 solid shell as contradicted by the stability of the con- 
 ditions which have been maintained during such long 
 past periods, and which exist at the present day. The 
 extent and character of volcanic action do not indicate 
 a condition of general instability in our earth, but one 
 
RECAPITULATION OF SEVERAL THEORIES. 349 
 
 of stability subject to small and local interferences. 
 The grandest volcanic disturbances appear small and 
 insignificant, if we take into account the vast dimen- 
 sions of the globe upon which they are displayed. 
 
 If, on the other hand, we consider the outer solid 
 shell to be of great thickness, we are met by the diffi- 
 culty of accounting for the upheaval of liquid matter 
 through such vast thicknesses of a solid shell. The 
 differences in character of lavas extruded from closely 
 adjoining volcanic districts seem equally difficult of 
 explanation on any theory of a central, fluid nucleus 
 and a solid, outer shell. Nor is the distribution of heat 
 within the earth's crust so uniform as might be antici- 
 pated, if the source of that heat be a great central 
 mass of highly heated materials. 
 
 Under these circumstances, geologists and physicists 
 have enquired whether any other conditions can be 
 imagined as existing in the earth's interior, which 
 would better account for the observed phenomena than 
 does the hypothesis of a liquid nucleus and a solid 
 outer shell. Two such alternative hypotheses nave 
 been suggested. 
 
 Mr. Hopkins, adopting the theory that the earth 
 has solidified both at the centre and its outer surface, 
 endeavoured to explain the occurrence of volcanoes and 
 earthquakes by supposing that cavities of liquid mate- 
 rial have been left between the solid nucleus and the 
 solid shell, and these cavities full of liquid materials 
 constitute the sources from which the existing volcanoes 
 
350 VOLCANOES. 
 
 of the globe draw their supplies. But this hypothesis 
 is found to be beset with many difficulties when we 
 attempt to apply it to the explanation of the pheno- 
 mena of volcanic action. It entirely fails, among other 
 things, to account for the remarkable fact that during 
 past geological periods the scene of volcanic action has 
 been continually shifting over the surface of the earth, 
 so that there is probably no considerable area of our 
 globe which has not at one time or other been invaded 
 by the volcanic forces. 
 
 By some other theorists, who have felt the full force 
 of this last objection, an attempt has been made to 
 explain the phenomena of volcanoes by supposing that 
 the globe is solid from its surface to its centre, but 
 that the internal portions of the globe are at such a 
 high temperature that they are only retained in a solid 
 condition by the enormous pressure to which they are 
 subjected. The central masses of the globe are thus 
 regarded as being in an actually solid, but in a poten- 
 tially liquid condition, and any local relief of pressure 
 is at once followed by the conversion of solid to 
 liquefied materials, in the district where the relief 
 takes place, resulting in the manifestation of volcanic 
 phenomena at the spot. It may be granted that this 
 hypothesis better accords with the known facts of Vul- 
 canology than any of those which we have previously 
 described, but it is impossible to shut our eyes to the 
 fact that not a few serious difficulties still remain. 
 Thus it is based upon the assumption that the law of 
 
DIFFICULTIES NOT YET EXPLAINED. 351 
 
 the elevation of the point of fusion by pressure is true 
 at temperatures and pressures almost infinitely above 
 those at which we are able to conduct observations ; 
 but neither experiment nor analogy warrant this con- 
 clusion, for the former shows that the elevation of the 
 point of fusion by pressure goes on in a continually 
 diminishing ratio, and the latter furnishes us with the 
 example of volatile liquids which, above tlieir critical 
 points, obstinately remain in a gaseous condition under 
 the highest pressures. Nor is it easy upon this hypo- 
 thesis to account for the very irregular distribution of 
 temperatures within the earth's crust, as demonstrated 
 by observations in mines, wells, and borings. The hy- 
 pothesis further requires the assumption that, at such 
 very moderate depths as are required for the reservoirs 
 of volcanoes, the effects of pressure and temperature on 
 the condition of rock-materials are so nicely balanced 
 that the smallest changes -at the surface lead to a dis- 
 turbance of the equilibrium. 
 
 It is the weight of these several objections that has 
 led geologists in recent years to regard with greater 
 favour those hypotheses which seek to account for the 
 production of high temperatures within parts of the 
 earth's crust, without having recourse to a supposed 
 incandescent nucleus. If it can be shown that there 
 are any chemical or mechanical forces at work within 
 the crust of the globe which are capable of producing 
 local elevations of temperature, then we may conceive 
 of a condition of things existing in the earth's interior 
 
352 VOLCANOES. 
 
 which is free from the objections raised by the astro- 
 nomer on the score of the earth's proved rigidity, and 
 by the geologist on the ground of its general stability, 
 and which at the same time seems to harmonise better 
 with the observed facts of the distribution of tempera- 
 ture within the earth's crust. How far the existence 
 of such chemical* and mechanical agencies capable of 
 producing high temperatures within the crust of the 
 globe have been substantiated, we have already en- 
 deavoured to point out. 
 
 It must be admitted, then, that the questions of 
 the nature of the earth's interior and the cause of the 
 high temperatures which produce volcanic phenomena, 
 are still open ones. We have not yet got beyond the 
 stage of endeavouring to account for the facts observed 
 by means of tentative hypotheses. Some of these, as 
 we have seen, agree with the facts, so far as they are 
 at present known, much better than others ; but the 
 decision between them or the rejection of the whole of 
 them in favour of some new hypothesis, must depend 
 on the results of future observation and enquiry. 
 
 It may be well, before leaving this subject, to re- 
 mark that they are all equally reconcilable with the 
 nebular theory of Kant and Laplace. Granting that 
 the matter composing our globe has passed successively 
 through the gaseous and liquid conditions, it is open 
 to us to imagine the earth as now composed of a liquid 
 nucleus with either a thick or a thin solid shell ; of a 
 solid nucleus and a solid shell with more or less liquid 
 
CAUSE OF THE PRESENCE OF WATEK IN LAVAS. 353 
 
 matter between them ; or, lastly, to conceive of it as 
 having become perfectly solid from the centre to the 
 surface. 
 
 But it is not upon the existence of a high tempera- 
 ture within certain parts of the earth's crust that the 
 production of volcanic activity alone depends. The pre- 
 sence of water and other liquid and gaseous substances 
 in a state of the most intimate admixture with the 
 fused rock-masses, is, as we have seen, the main cause 
 of the violent displays of energy exhibited at volcanic 
 centres. And we shall now proceed to notice the hy- 
 potheses which have been suggested to account for the 
 presence of these liquid and gaseous bodies in the midst 
 of the masses of incandescent materials poured out from 
 volcanic vents. 
 
 There is an explanation of this presence of water 
 and various gases in the masses of molten rock-materials 
 within the earth's crust which at once suggests itself, 
 and which was formerly very generally accepted. Vol- 
 canoes, as we have seen, are usually situated near coast- 
 lines, and if we imagine fissures to be produced by 
 which sea-water finds access to masses of incandescent 
 rock-materials, then we can regard volcanic outbursts 
 as resulting from this meeting of water with rock- 
 masses in a highly heated condition. This supposition 
 has been thought to receive much support from the 
 fact that many of the gases evolved from volcanic vents 
 are such as would be produced by the decomposition of 
 substances present in sea-water. 
 
354 VOLCANOES. 
 
 But it frequently happens that an explanation which 
 at first sight appears to be very simple and obvious, 
 turns out on more critical examination to be quite the 
 reverse, and this is the case with the supposed origina- 
 tion of volcanic outbursts by the access of sea-water to 
 incandescent rock-material by means of earth-fissures. 
 It is difficult to understand how, by such means, that 
 wonderfully intimate union between the liquefied rock 
 and the water, evolved in such quantities during vol- 
 canic outbursts, could be brought about; and moreover, 
 we can scarcely regard the production of fissures in the 
 earth's crust as being at the same time both the cause 
 and the effect of this influx of water to the deep-seated 
 rock-masses at a high temperature. 
 
 During recent years the attention of both geologists 
 and physicists has been directed to a remarkable pro- 
 perty exhibited by many liquids and solids, as supplying 
 a possible explanation of the phenomena of volcanic 
 action. The property to which we refer is that whereby 
 some liquid and solid substances are able to absorb 
 many times their volume of certain gases - which gases 
 under different conditions may be given off again or 
 6 occluded ' from the liquids or solids. This power of 
 absorption is a very remarkable one ; it is not attended 
 with chemical combination, but the amount of condensa- 
 tion which gases must undergo within the solid or liquid 
 substances is sometimes enormous. Water may be made 
 to absorb more than 1,000 times its volume of ammonia, 
 and more than 500 times its volume of hydrochloric 
 
ABSOKPTION OF GASES BY LIQUIDS AND SOLIDS. 355 
 
 acid. Alcohol may absorb more than 300 times its 
 volume of sulphurous acid. Charcoal may absorb 100 
 times its volume of ammonia, 85 times its volume of 
 hydrochloric acid, 65 times its volume of sulphuretted 
 hydrogen, 55 times its volume of sulphurous acid, and 
 35 times its volume of carbonic acid. Platinum-black 
 absorbs many times its volume of oxygen and other 
 gases. 
 
 This power of absorption of gases varies in different 
 solids and liquids according to the conditions to which 
 they are subjected. Dr. Henry showed it to be a general 
 law in liquids that, as the pressure is augmented, the 
 weight of the gas absorbed is proportionately increased. 
 
 Sometimes this absorption of gases takes place only 
 at high temperatures. Thus silver in a state of fusion 
 is able to absorb 22 times its volume of oxygen gas. 
 When the metal is allowed to cool this gas is given 
 off, and if the cooling takes place suddenly a crust is 
 formed on the surface, and the phenomenon known as 
 the ' spitting of silver ' is exhibited. Sometimes during 
 this operation miniature cones and lava-streams are 
 formed on the surface of the cooling mass, which pre- 
 sent a striking resemblance to those formed on a grand 
 scale upon the surface of the globe. Similar pheno- 
 mena are exhibited by several other metals and by the 
 oxide of lead. 
 
 The researches of Troost and others have shown 
 that molten iron and steel possess the property of 
 absorbing considerable quantities of oxygen, hydrogen, 
 
356 VOLCANOES. 
 
 carbonic acid, and carbonic oxide, and that these gases 
 are given off in the operation known as ' seething,' when 
 either the pressure or the temperature is diminished. 
 
 Hochstetter has shown that in the process of extract- 
 ing sulphur from the residues obtained during the 
 manufacture of soda, some very interesting phenomena 
 are manifested. The molten sulphur is exposed to a 
 temperature of 262 Fahrenheit, and a pressure of two 
 or three atmospheres, in the presence of steam ; under 
 these circumstances it is found that the sulphur absorbs a 
 considerable quantity of water, which is given off again 
 with great violence from the mass as it undergoes 
 solidification. The hardened crust which forms on the 
 surface of the molten sulphur is agitated and fissured, 
 miniature cones and lava-streams being formed upon 
 it, which have a striking resemblance to the grander 
 phenomena of the same kind exhibited upon the crust 
 of the globe. 
 
 The observations which we have described prove con- 
 clusively that many liquids and solids in a molten con- 
 dition have the power of absorbing many times their 
 volume of certain gases, and that this action is aided 
 by heat and pressure. 
 
 That the molten materials which issue from volcanic 
 vents have absorbed enormous quantities of steam and 
 other gases, we have the most undisputable evidence. 
 The volume of such gases given off during volcanic 
 outbursts, and while the lava-streams are flowing and 
 consolidating, is enormous, and can only be accounted 
 
SOUKCE OF THE ABSORBED GASES. 357 
 
 for by supposing that the masses of fluid rock have 
 absorbed many times their volume of the gases. But 
 we have another not less convincing proof of the same 
 fact in the circumstance that volcanic materials which 
 have consolidated under great pressure such as 
 granites, gabbros, porphyries, c. exhibit in their 
 crystals innumerable cavities containing similar gases 
 in a liquefied state. 
 
 It is to the violent escape of these gases from the 
 molten rock-masses, as the pressure upon them is re- 
 lieved, that nearly all the active phenomena of volcanoes 
 must be referred ; and it was the recognition of this fact 
 by Spallanzani, while he was watching the phenomena 
 displayed in the crater of Stromboli, which laid the 
 foundations of the science of Vulcanology. 
 
 But here another question presents itself to the 
 investigator of the phenomena of volcanoes : it is this. 
 At what period did the molten rock-masses issuing from, 
 vents absorb those gaseous materials which are so 
 violently oeelttd^d- from their midst during eruptions ? 
 Two different answers to this question have been sug- 
 gested. It may be that the original materials of which 
 our globe was composed consisted of metallic substances 
 in a state of fusion which had absorbed many gases, 
 and that, in the fluid masses below the solid crust, vast 
 quantities of vapour and gas are stored up, which are 
 being gradually added to the atmosphere during vol- 
 canic outbursts. The fact that meteorites, which, as 
 we have seen, in all probability closely resemble the 
 
358 VOLCANOES. 
 
 materials forming the earth's interior, sometimes yield 
 many times their volume of hydrogen and other gases, 
 may be thought to lend some support to this idea. If 
 it be the correct one, we must regard our globe as 
 gradually parting with its pent-up stores of energy, in 
 those absorbed gases and vapours held in bondage by 
 the solid and fluid materials of its interior. 
 
 But there is another hypothesis which is, to say 
 the least, equally probable. Water containing various 
 gases in solution is continually finding its way down- 
 wards by infiltration into the earth's crust. Much of 
 this water, after passing through pervious beds, reaches 
 some impervious stratum and is returned to the surface 
 in the form of springs. But that some of this perco- 
 lating water penetrates to enormous depths is shown by 
 the fact that the deepest mines and borings encounter 
 vast underground supplies of water. When we re- 
 member that nearly three-fourths of the earth's surface 
 is covered by the waters of the ocean, and that the 
 average depth of these oceanic waters is more than 
 10,000 feet, we may easily understand how great a 
 portion of the earth's crust must be penetrated by 
 infiltrating waters which can find no outlet in springs 
 The penetration of the waters of the ocean into the 
 earth's crust will be aided, too, by the enormous pres- 
 sure amounting to not less than several tons to the 
 square-inch upon the greater part of the ocean-floor. 
 It might be thought that this downward penetration 
 of water would be counteracted by the upward current 
 
POSITION OF THE ISOGEOTHERMS. 359 
 
 of steam that would be produced as these subterranean 
 waters reach the hotter portions of the earth's crust. 
 But the experiments of Daubree have conclusively shown 
 that the penetration of water through rocks takes place I 
 in opposition to the powerful pressure of steam in the 
 contrary direction. Hence, we may assume that cer- 
 tain quantities of water, containing various gases and 
 solids in solution, are continually finding their way by 
 capillary infiltration from the surface to the deeply 
 seated portions of the earth's crust, there to undergo 
 absorption by the incandescent rock-masses and to pro- 
 duce oxidation of some of their materials. 
 
 The deep-sea soundings of the ' Challenger ' have 
 shown that the floor of the ocean is constantly main- 
 tained at a temperature but little above that of the 
 freezing point of water. This low temperature is pro- 
 bably produced by the absorption of heat from the 
 earth's crust by the waters of the ocean, which distribute 
 it by means of convection currents on the grandest 
 scale. Hence, the isogeotherms, or lines indicating the 
 depths at which the same mean temperature is found 
 within the earth's crust, are probably depressed beneath 
 the great ocean-floors, and rise towards the land-masses. 
 It is to this circumstance, combined with that of the 
 enormous pressure of water on the ocean-beds, that we 
 must probably ascribe the general absence of volcanoes 
 in the deep seas and their distribution near coast- 
 lines. 
 
 We have thus briefly reviewed the chief hypotheses 
 
360 VOLCANOES 
 
 which have been suggested in order to account for the 
 two great factors in all volcanic phenomena namely, 
 the presence of highly heated rock-masses within the 
 earth's crust, and the existence of various vapours and 
 gases in a state of most intimate mechanical, but not 
 chemical, union with these incandescent materials. It 
 must be admitted that we do not at present appear to 
 have the means for framing a complete and consistent 
 theory of volcanic action, but we may hopefully look 
 forward to the time when further observation and ex- 
 periment shall have removed many of the existing 
 difficulties which beset the question, and w r hen by the 
 light of such future researches untenable hypotheses 
 shall be eliminated and the just ones improved and 
 established. 
 
 But if we are constrained to admit that a study of 
 the observed phenomena and established laws of volcanic 
 action have not as yet enabled us to frame any complete 
 and satisfactory theory on the subject, we cannot lose 
 sight of the fact that all modern speculation upon this 
 question appears to be tending in one definite direction. 
 It is every day becoming more and more clear that our 
 earth is bound by ties of the closest resemblance to the 
 other members of that family of worlds to which it 
 belongs, and that the materials entering into their con- 
 stitution, and the forces operating in all are the same. 
 
 We have had occasion in a previous chapter to point 
 out that there are the strongest grounds for believing 
 the interior of our globe to consist of similar materials 
 
ERUPTIVE ACTION IN THE SUN. 361 
 
 to those found in the small planetary bodies known as 
 meteorites. That the comets are merely aggregations 
 of such meteorites, and that the planets differ from 
 them only in their greater dimensions, may be regarded 
 as among the demonstrated conclusions of the astrono- 
 mer. The materials found most abundantly in me- 
 teorites and in the interior of our globe are precisely 
 the same as those which are proved to exist in an in- 
 candescent state in our sun. Hence we are led to 
 conclude that the whole of the bodies of the solar sys- 
 tem are composed of the same chemical elements. 
 
 That the forces operating in each of these distant 
 bodies present striking points of analogy is equally 
 clear. The sun is of far greater dimensions than our 
 earth, and is still in great part, if not entirely, in a 
 gaseous condition. The great movements in the outer 
 envelopes of the sun exhibited in the ' sun-spots ' and 
 ' solar prominences/ recall to the mind the phenomena 
 of volcanic activity upon our globe. But the vast 
 energy still existing in the intensely heated mass of 
 the sun, and the wonderful mobility of its gaseous 
 materials, give rise to appearances beside which all 
 terrestrial outbursts seem to sink into utter insignifi- 
 cance. Vast cavities of such dimensions that many 
 globes of the size of our earth might be swallowed up 
 in them are formed in the solar envelopes in the 
 course of a few days or hours. Within these cavities 
 or sun-spots incandescent vapours are observed, rush- 
 ing upwards and downwards with almost inconceivable 
 velocity. 
 
362 
 
 VOLCANOES. 
 
 The drawings made by Secchi, and reproduced in 
 figs. 89 and 90, will give some idea of the appearances 
 presented by these great holes in the solar enve- 
 lopes. 
 
 In fig. 89 a group of sun-spots is represented and, 
 in their circular outlines and tendency to a linear ar- 
 
 FIG. 89. A GUOUP OF SUN-SPOTS. (After Secchi.) 
 
 rangement, they can scarcely fail to remind anyone 
 familiar with volcanic phenomena of terrestrial craters, 
 though their dimensions are so much greater. 
 
 In fig. 90 the sun-spot represented shows the pre- 
 sence of large floating masses of incandescent materials 
 rushing upwards and downwards within the yawning 
 gulf. 
 
PHENOMENA OF SUN-SPOTS 
 
 363 
 
 FIG. 90. A SUN-SPOT, SHOWING THE GREAT MASSES OF INCANDESCENT 
 
 VAPOUR RISING OR FALLING WITHIN IT. (After Secchi.) 
 
 From fig. 91, taken from 
 a drawing by Mr. Norman 
 Lockyer, we may under- 
 stand the movements of 
 these great protuberances of 
 incandescent gas which are 
 seen on the sides of the 
 sun-spots. 
 
 The so-called solar pro- 
 minences present even more 
 striking resemblances to the volcanic outbursts of our 
 globe. 
 
 FIG. 91. THE EDGE OF A SUN-SP* >T, 
 SHOWING A PORTION OF THE PRO- 
 MINENT MASSES OF INCANDESCENT 
 
 GAS (A), WHICH DETACHED IT- 
 SELF AT E AND FLOATED INTO 
 THE MIDST OF THE CAVITY. 
 
364 
 
 VOLCANOES. 
 
 Two drawings made by Mr. Norman Lockyer will 
 serve to give some idea of the vast dimensions of these 
 solar prominences, and of the rapid changes which take 
 place in their form. 
 
 FIG. 92. DRAWING OF A SOLAR PROMINENCE, MADE BY MR. NORMAN 
 LOCKYER ON MARCH 14, 1869, AT 11 H. SM. A.M. 
 
 The masses of incandescent gas were estimated as 
 being no less than 27,000 feet in height, yet in ten 
 minutes they had totally changed their form and ap- 
 pearance, as shown in fig. 93. 
 
 Even still more striking are the changes recorded 
 
SOLAR PROMINENCES. 
 
 365 
 
 by Professor Young, of New-Haven, in a solar pro- 
 minence, which he observed on September 7, 1871. 
 
 That astronomer described a mass of incandescent 
 gas rising from the surface of the sun to the height 
 
 FIG. 93. THE SAME OBJECT, AS SEEN AT 11 H. 15>i. ON THE SAME DAY. 
 
 of 54,000 miles. In less than twenty-five minutes he 
 saw the whole mass torn to shreds and blown upwards, 
 some of the fragments being in ten minutes hurled to 
 the height of 200,000 miles above the sun's surface. 
 The masses of incandescent gas thus hurled upwards 
 were of enormous dimensions, the smallest being esti- 
 mated as having a greater area than the whole of the 
 
366 
 
 VOLCANOES. 
 
 
EXTINCT VOLCANOES OF THE MOON. 367 
 
 British Islands, and the force with which they were 
 urged upwards was so great that they acquired a velocity 
 of 166 miles per second. The accompanying woodcut 
 shows the successive appearances presented by this 
 grand eruptive outburst on the surface of the sun. 
 
 The moon, which is of far smaller size than our 
 earth, exhibits on its surface sufficiently striking evi- 
 dences of the action of volcanic forces. Indeed the 
 dimensions of the craters and fissures which cover the 
 whole visible lunar surface are such that we cannot but 
 infer volcanic activity to have been far more violent 
 on the moon than it is at the present day upon the 
 earth. This greater violence of the volcanic forces on 
 the moon is perhaps accounted for by the fact that the 
 force of gravity on the surface of the moon is only 
 one-sixth of that at the surface of the earth ; and 
 thus the eruptive energy will have a much less smaller 
 resistance to overcome in bursting asunder the solid 
 crust and accumulated heaps of ejected materials on its 
 surface. But the volcanic action on the moon appears 
 now to have wholly ceased, and the absence of both 
 water and atmosphere in our satellite suggests that this 
 extinction of volcanic energy may have been caused by 
 the complete absorption of its- gaseous envelope. The 
 appearance presented by a portion of the moon's sur- 
 face is shown in fig. 95. 
 
 The sun and the moon appear to exhibit two 
 widely separated extremes in the condition assumed 
 during the cooling down from a state of incandescence 
 
368 
 
 VOLCANOES. 
 
 of great globes of vaporised materials. The several 
 planets, our own among the number, probably exhibit 
 various intermediate stages of consolidation. 
 
 FIG. 95. A GROUP OF LUNAR CRATERS (MAUROLYCUS, BAROCIUS, ETC.). 
 
 THE LARGEST BEING MORE THAN 60 MILES IN DIAMETER. 
 
 Our earth is, as we have seen, closely allied to the 
 other bodies of the solar system in its movements, its 
 relations, and its composition ; and a true theory of 
 
EEUPTIVE ACTION IN THE SUN, EAKTH AND MOON. 369 
 
 terrestrial vulcanicity, when it is discovered, may be 
 expected not only to afford an explanation of the phe- 
 nomena displayed on our own globe, but to account for 
 those displays of internal energy which have been 
 manifested in other members of the same great family 
 of worlds. 
 
 17 
 
INDEX. 
 
 [The subjects illustrated in the engravings are indicated by italics, the names of 
 authors are in CAPITALS.] 
 
 ABI 
 
 ABICH, cited, 122 
 ** researches of, 4 
 Absorption of gases by liquids 
 
 and solids, 354, 355 
 Acid lavas, 48 
 
 ^Eolian Islands. Ste Lipari Is- 
 lands 
 
 jEolus, origin of myth, 35 
 Africa, volcanoes of, 227 
 South, diamonds of, 147 
 Agates, formation of, 150 
 ALLPORT, Mr., cited, 259 
 Alps, formation of, 292 
 Altered lavas, names given to, 
 
 261 
 
 America, volcanoes of, 227 
 Amygdaloids, formation of, 140, 
 
 141 
 
 Andesites, 50, 59 
 Andesite-volcanoes, 126 
 ANDEEWS, Professor, cited, 321 
 Anne Boleyn and Etna, 3 
 ARMSTRONG-, Sir W., hydro-elec- 
 tric machine, 29 
 Arthur's Seat, 275 
 Artificial stone, 55 
 Asia, volcanoes of, 227 
 Asiderites, 316 
 Asmanite, 314 
 Astroni, crater-ring of, 170 
 Atlantic, volcanoes in, 228 
 
 BOR 
 
 Auvergne, breached cones of, 123, 
 fig. 40 
 
 denuded cones in, 124, fig. 42 
 
 incrusting springs of, 1 84 
 
 puys of, 152, 212 
 
 volcanic cones in, 79 
 
 DALL-AND-SOCKET structure 
 1 in basaltic columns, 107 
 
 Barrancos, formation of, 209 
 
 Basalt, controversy concerning 
 origin of, 249 
 
 Basalts, 49, 50, 59 
 
 Basaltic columns of Bohemia, 
 107 
 
 of Central Germany, 107 
 
 of Monte Albano, 107 
 
 from the Giants Cause- 
 may, 107, fig. 29 
 
 Basic lavas, 48 
 
 Bath, hot spring of, 219 
 
 Ben Nevis, 274 
 
 Bohemia, volcanoes of, 126 
 
 lavas of, 103 
 
 Boiling. See Ebullition 
 
 BONNEY, Professor, cited, 69, 
 109, 259 
 
 Boracic acid at volcanic vents, 
 216 
 
INDEX. 
 
 BOU 
 
 ,M-bon, volcano of, 176, figs. 74, 
 
 75 
 Bracciano, crater-lake of, 178, 
 
 fig. 77 
 
 Breached cones, 123, fig. 40 
 Bubbles of steam, escape from 
 
 lava, 21 
 Bubbles, spontaneous movement 
 
 of, in liquid cavities, 62, fig. 8 
 
 cause of, 65 
 
 BUCH, VON, researches of, 4 
 Buda-Pest, deep well of, 335, 
 
 341 
 
 Biidos Hegy, Transylvania, 215 
 BUNSEN, cited, 201 
 Burning, does not take place at 
 
 volcanoes, 2 
 
 nADEK IDRIS, 274 
 
 ^ ' Calderas,' formation of, 180 
 
 Caldera of Palma, 209 
 
 Cambro-Silurian volcanoes of 
 British Islands, 274 
 
 Campi-Phlegrcei, map of, fig. 11 
 
 volcanoes of, 79 
 
 tuff -cones of,* 118 
 
 fissures in, 197 
 
 Carbonic acid in casdties of crys- 
 tals, 63 
 
 Carboniferous volcanoes of Brit- 
 ish Islands, 275 
 
 Carlsbad, Strudel of, 218 
 
 Strudelstein of, 184 
 Caspian Sea, mud-volcanoes of, 
 
 182 
 Catacecaumene, volcanic cones 
 
 in, 79 
 Cause of proximity of volcanoes 
 
 to sea, 239 
 Central Asia, volcanoes of, 236 
 
 America, mud-volcanoes, 182 
 Pacific, volcanoes of, 236 
 
 4 Challenger,' H.M.S., voyage of, 
 73 
 
 soundings of, 359 
 
 CHANCE, Messrs., of Birming- 
 ham, 55 
 
 CEA 
 
 Charnwood Forest, ancient vol- 
 canic rocks of, 259 
 
 Chemical deposits at Vulcano, 
 44 
 
 on surfaces of lavas, 110 
 
 elements present in lavas, 46 
 
 theory of volcanoes, 344, 345 
 Chiaja di Luna, 108 
 Chimborazo, size of, 44 
 
 151 
 
 Chodi-Berg, Hungary, 161 
 Citlaltepetl, mem of , 169, fig. 69 
 Coast-lines, proximity of volca- 
 noes to, 228 
 
 COLE, Mr. GrRENVILLE, 110 
 
 Colours of lavas, 49 
 Columns in lava, 105 
 dimensions of, 105 
 
 radiating in intrusive masses, 
 136 
 
 Columnar structure in lavas, 104 
 
 origin of, 105 
 
 Columnar lava-stream on the Ar- 
 
 deche, 107, fig. 28 
 Combustion, does not lake place 
 
 at volcanoes, 2 
 Composite cones, 128, 161 
 Comstock mines, temperature of, 
 
 342 
 Concentric jointing in lava, 108, 
 
 fig. 30 
 Cones composed of viscid lava, 
 
 129, fig. 43 
 - miniature on lava-streams, 
 
 100, 101, fig. 25, 26 
 
 natural sections of, 129 
 
 shifting of axis in, 167 
 Coolin Hills, Skye, 144 
 Cotopaxi, volcanic dust of, 69 
 
 view of, 168, fig. 68 
 Craters, formation of, 82 
 
 origin of, 167 
 
 position of, 167 
 
 fissuring of sides, 180 
 Crater of Stromboli, aperture at 
 
 bottom of, 15 
 Crater-lakes, formation of, 171 
 
 of Agnano, 171 
 
 of Albano, 171 
 
INDEX. 
 
 373 
 
 CRA 
 
 Crater-lakes of Avernus, 171 
 
 ofaffno,l71,fig. 71 
 
 of Bolsena, 171 
 
 of Bracciano, 171 
 
 of Frascati, 173, 175 
 
 of Gfustavila, 171, fig. 72 
 
 of Laach, 171 
 
 of Nemi, 171 
 Crater-rings, formation of, 170 
 Crater-ring of Somma, 177, fig. 76 
 Crater-ring of Pianura, 174 
 
 of Piano di Quarto, 174 
 
 of Vallariccia, 174 
 
 Creuzot, shafts at, 340 
 
 ' Critical point ' of liquids, 63 
 
 Crust of globe, definition of, 308 
 
 Crystals in lavas, 51 
 
 formed of crystallites, 
 
 54-57 
 
 formed in subterranean 
 
 reservoirs, 60 
 
 interruption in growth of, 
 
 60 
 
 pressure under which formed, 
 65 
 
 deposited on surface of lava, 
 110 
 
 porphyritic, origin of, 256 
 
 Crystalline minerals formed be- 
 neath volcanoes, 146, 147 
 
 ejected from volcanoes, 
 
 147 
 
 Crystallised minerals of volca- 
 noes, 46 
 
 Crystallites, aggregates of, 54, 
 Frontispiece 
 
 Crystallites in lavas, 53, Fron- 
 tispiece 
 
 Crypto-crystalline base, 57 
 
 ' Cupolas,' 135 
 
 Curral of Madeira, 209 
 
 DACITES, 198 
 
 L ' DAN A, Prof essor,J.D., cited, 
 100, 159, 291, 301, 327, 338, 
 339 
 
 DARWIN, Mr., cited, 245, 246, 
 271, 289 
 
 ELE 
 
 DAUBENY, cited, 182 
 DAUBREE, M., cited, 147, 315, 
 
 320, 358 
 
 Daubreelite, 314 
 DAVY, Sir HUMPHRY, chemical 
 
 theory of volcanoes, 344, 345 
 Deccan of India, 103 
 Density of the earth, 308 
 Denuded cones and craters, 158, 
 
 fig. 59 
 
 Denudation, effects of, on volca- 
 noes, 114 
 Deposits about volcanic fissures, 
 
 42 
 
 Detonations at Vesuvius, 26 
 Devonian volcanoes of British 
 
 Islands, 274 
 Diorite, 59 
 
 DOLOMIEU, cited, 4, 39 
 DUROCHER, cited, 201 
 Dykes, formation of, 116, 117, 
 
 209, 210 
 
 structure of rock in, 211 
 
 pseudo-, 119 
 
 Dynamical theory of volcanoes, 
 347, 348 
 
 "L^ARTH'S interior, nature of, 
 ** 309 
 
 physical condition of, 325 
 
 - hypothesis concerning, 
 328-330 
 
 rela'ion to other planets, 310, 
 311 
 
 Earthquakes, depth of origin of, 
 343, 344 
 
 connection with volcanoes, 343 
 
 accompanying Vesuvian erup- 
 tion of 1872, 27 
 
 Ebullition, compared to volcanic 
 
 eruptions, 19, 20 
 Eifel, volcanic cones of, 45 
 Ejected blocks, 45 
 
 materials, height to whijh 
 thrown, 72 
 
 stratification of, 117-119 
 Elements, pyroxenic and trachy- 
 tic, theory of, 201 
 
INDEX. 
 
 ELE 
 
 Cation-craters, theory of, 135, 
 
 209 
 Erroneous opinions, sources of, 
 
 in regard to volcanoes, 2 
 Eruptions, feeble and violent 
 
 compared, 31 
 
 prediction of, not possible, 32 
 
 intervals between, 33 
 
 of varying intensity, 33 
 
 and barometric pressure, 36 
 
 effects of repetition of from 
 same fissure, 80 
 
 Eruptive action in sun and 
 moon, 360-369 
 
 Etna, ideas of ancients concern- 
 ing, 3 
 
 and Anne Boleyn, 3 
 
 observatory on, 37 
 
 size of, 44 
 , 151 
 
 eruptions at summit and on 
 flanks, 207 
 
 Etna, dyke and lava-stream in, 
 
 133, fig. 54 
 Etna, views of, 162, 163, figs. 62, 
 
 63 
 Euganean Hills, 139 
 
 volcanof-s of, 201 
 Europe, volcanoes of, 227 
 Extra-terrestrial rocks, 316 
 Extra-terrestrial rocks, relation 
 
 to ultra-basic rod's, 322, fig. 88 
 
 pELSTONES, 263 
 
 Ferric-chloride, mistaken for 
 
 sulphur, 41 
 Fissure on flanks of Etna, 194, 
 
 fig. 84 
 
 Fissure-eruptions, 188 
 Fissures, volcanic cones on, 194 
 
 systems of, 198 
 Fingal's Cave, 106 
 
 Flames, phenomena mistaken 
 for, 2 
 
 at volcanoes, feebly luminous, 
 17 
 
 false appearance of, in volca- 
 noes, 17 
 
 Flames at volcanic vents, 41 
 Flashing light-house, compared 
 
 to Stromboli, 10 
 Floods, accompanying volcanic 
 
 outbursts, 30 
 FORBES, Mr. DAVID, cited, 337, 
 
 339 
 Fossils, from beneath Vesuvius, 
 
 45 
 
 supposed in basalt, 250 
 FOUQUE, M., cited, 110, 213 
 Fumaroles, gases emitted from, 
 
 213 
 
 Fusiyama, form of, 90, 166 
 Fusiyama, 178, fig. 77 
 
 riABBEO, 59 
 
 ^* Gardiner's river, travertine 
 
 terraces of, 185 
 Gases emitted from volcanoes, 
 
 40 
 
 volcanic vents, 212-216 
 
 Geanticlinals, formation of, 297 
 Gems, formation of, 147 
 
 mode of occurrence, 148 
 Geological continuity, doctrine 
 
 of, 247 
 
 Geosynclinals, formation of, 294 
 Geysers, formation of, 217 
 
 intermittent action of, 218 
 
 of Colorado, 184, 217 
 - of Iceland, 184, 217 
 Giant's Causeway, 108 
 GILBERT, Mr. G. K., cited, 208 
 Girgenti, mud-volcanoes of, 182 
 Glass, formed by fusion of lavas, 
 
 52 
 
 Glasses, composed of certain sili- 
 cates, 58 
 
 Glassy base, 57 
 
 GOETHE, cited, 112 
 
 Graham Isle, 178, 179, fig. 78 
 
 GRAHAM, cited, 345 
 
 Grand Sarcoui, Auvergne, 161 
 
 Granite, 59 
 
 Granite of Secondary and Ter- 
 tiary ages, 254 
 
INDEX. 
 
 375 
 
 GRA 
 
 Granitic rocks, position beneath 
 
 volcanoes. 145 
 Great earth movements, nature 
 
 of, 286 
 Great volcanic bands of the 
 
 globe, 232-234 
 Grenelle, boring of, 341 
 Grey stones, 49 
 Groundmass of lavas, 52 
 Grotto del Cane, 215 
 Guevo Upas, Java, 215 
 GUISCABDI, Professor, referred 
 
 to, 45 
 Gustavila, crater -lake of y 172, 
 
 fig. 72 
 
 TJAMILTON, Sir W., researches 
 
 11 of, 4, 75, 84 
 
 observations on Vesuvius, 
 
 80 
 
 HANNAY, Mr., referred to, 147 
 HAETLEY, Mr. NOEL, referred 
 
 to, 65 
 Hawaii, volcanoes of, 100, 125 
 
 lava-masses of, 159 
 
 volcanic eruptions at different 
 levels, 327 
 
 Hebrides, volcanoes of, 271 
 
 HENEY, Dr., cited, 355 
 
 Henry Mountains, Southern 
 
 Utah, 208 
 
 Hephaestus, forge of, 3 
 HOCHSTETTEE, cited, 135, 356 
 Holosiderites, 315 
 HOPKINS, Mr., cited, 349 
 Hot springs, numbers of, 219 
 Humboldt, researches of, 4 
 Hungary, lavas of, 96, 103 
 
 volcanoes of, 126, 201 
 
 deep wells of. 341 
 Hverfjall, Iceland, 178, fig. 77 
 Hydro-electric machine of Sir 
 
 W. Armstrong. 29 
 Hypothesis, value of, 331-333 
 
 ICE under lava of Vesuvius in 
 1872, and of Etna, 110 
 
 LAK 
 
 Iceland, volcanic dust of, carried 
 
 to Norway, 72 
 
 Indian Ocean, volcanoes in, 229 
 Insel Ferdinandez, 178, 179, fig. 
 
 78 
 
 Intermediate lavas, 48 
 Intervals between eruptions, 33 
 Ireland, north-east of, 103 
 Iron in Ovifak-basalts, 319, fig. 
 
 87 
 Iron, seething of, 356 
 
 of Ovifak, terrestrial origin 
 of, 320 
 
 Ischia, eruption in 1301, 164 
 
 crater -lake of JBagno in, 172, 
 fig. 71 
 
 plan of, 163, fig. 64 
 
 parasitic cones in, 164, fig. 65 
 Island of Bourbon, 93 
 
 Isle Julie, 178, 179, fig. 78 
 Isogeotherms, 359 
 
 TANSSEN, referred to, 42 
 " Joint-structures in lava, 
 104-110 
 
 JAMMERB UHL, 1 12-1 14, fig. 
 
 - - section of, 114, fig. 34 
 
 section in side of, 118, fig. 36 
 KANT, nebular hypothesis of, 
 
 352 
 Kilauea, volcano of, 71, 138 
 
 crater of, 181 
 
 KING, Mr. CLABBNCE, cited, 301 
 
 T AACHER S E E, minerals 
 
 *J ejected at, 149 
 
 Lac Paven, Auvergne, 171, fig. 
 
 70 
 
 ' Laccolites,' formation of, 208 
 Lago di Bolsena, 173, 175 
 Lago di Bracciano, dimensions 
 
 of, 172, 173 
 Lake Avernus, 215 
 
376 
 
 INDEX. 
 
 LAP 
 
 Lapilli, 70 
 
 LAPLACE, nebular hypothesis of, 
 
 325, 352 
 
 Lavas, action of acid gases on, 41 
 resemblance to slags, 46 
 chemical elements in, 46 
 
 oxygen in, 4.7 
 
 silicon in, 47 
 
 proportion of silica and other 
 oxides in, 47 
 
 silicates in, 47 
 
 acid, intermediate, basic, 48 
 
 specific gravities of, 49 
 
 colours of, 49 
 
 microscopic study of, 50 
 
 fusibility of, 51 
 
 minerals in, 51 
 
 artificially fused, 51 
 
 crystals in, 51, 93 
 
 ground mass of, 52 
 
 crystalline forms of, 59 
 
 of Bohemia, 103 
 
 of Hungary, 96, 103 
 
 of Kilauea, 95 
 
 of Lipari, 96 
 
 of Niedermendig, 103 
 
 of Vesuvius, 104 
 
 of Volvic, 95 
 
 of Vulcano, 95 
 
 presence of water in, 102 
 
 chemical deposits on, 1.10 
 
 different fluidity of, 204 
 
 augite and hornblende in, 267 
 Lara, cascade of, 93, fig. 18 
 Lava-cones, composed of liquid 
 
 lava, 125 
 of viscid lava, 126, 127 
 
 characters of, of liquid lava, 
 159 
 
 of viscid lava, 160 
 Lava-cones, outlines of, 160, fig. 60 
 Lava, in deep-seated reservoirs, 
 
 138 
 
 consolidation of, at great 
 depths, 139 
 
 Lava-fountains, 94 
 Lava-skeets, intrusive, 136, 137, 
 
 fig. 56 
 * Lava ' ornaments cf Naples, 45 
 
 MAD 
 
 < Lava,' slow-cooling of, 110 
 
 a bad conductor of heat, 110 
 
 ice under, 110 
 Lava-streams, nature of move- 
 ments, 92 
 
 difference in liquidity of, 92 
 
 miniature cones on, 100, 101 
 
 vast dimensions of, 102 
 
 structure of, 103 
 
 position of columns in, 106 
 
 sinking of surface of, 111 
 ' Lave di fan go/ 30 
 
 ' Lave di f uoco,' 30 
 
 Lawrenceite, 314 
 
 Laws of volcanic action, 38 
 
 LE COI^TE, cited, 347 
 
 Leucite, absence from ancient 
 
 lavas, 268 
 
 Lightning, accompanying vol- 
 canic outbursts, 28 
 Linear arrangement of volcanic 
 
 vents, 191 
 
 of volcanoes, 231 
 
 Lipari Islands, 3, 39 
 
 fissures in, 197 
 
 pumice-cones in, 154 
 
 order of appearance of 
 
 lavas in, 200 
 breached pumice-cone in, 
 
 124, fig. 41 
 
 map of, 192, fig. 81 
 
 lavas of, 96, figs. 20, 21 
 
 Liquids in cavities of crystals, 63 
 Liquid cavities in lavas, 60, fig. 7 
 spontaneous movement of 
 
 bubbles in, 62, fig. 8 
 spontaneous movement of 
 
 bubbles in, cause of, 65 
 LOCKYER, Mr. NORMAN, cited, 
 
 322, 363, 364 
 
 Lunar craters, 368, fig. 95 
 LYELL, Sir CHARLES, cited, 135, 
 
 167, 197 
 
 MACCULLOCH, cited, 207, 
 208 
 
 Madeira, cliff-section in, 128, fig. 
 47 
 
INDEX. 
 
 377 
 
 MAG 
 
 Magmas, theory of, 201 
 
 objections to, 202, 203 
 MALLET, Mr., cited, 269, 343, 346 
 Mamelons of Bourbon, 126, 127, 
 
 figs. 45, 46 
 MASKELYNE, Professor, cited, 
 
 314 
 Massa di Somma, destruction cf, 
 
 26 
 
 Mauna Loa, 138 
 Metamorphism around volcanic 
 
 vents, 145 
 Meteorites, nature of, 312 
 
 composition of, 313 
 
 minerals of, 314 
 
 classification of, 315 
 Melaphyres, 262 
 Miascite, 59 
 MICHEL LEVY, M., 110 
 Micro-crystalline base, 58 
 Microliths. See Crystallites 
 Microscopic study of lavas, 50 
 Minerals in lavas, 51 
 
 of Vesuvius, 46 
 Mineral-veins, formation of, ] 49 
 
 connection with volcanoes, 
 , 220 
 
 nature of materials in, 321 
 Misenum, Cape of, section of tuff- 
 cone of, 121, fig. 38 
 
 Modena, mud- volcanoes of, 182 
 Mont Dore, section at, 130, fig. 48 
 Monte Cerboli, Tuscany, 216 
 Monte Massi, Tuscany, well at, 
 
 341 
 
 Monte Nuovo, history of forma- 
 tion of, 76 
 
 description of, 77, 78, fig. 10 
 
 152 
 
 crater of, 168 
 
 production of fissure at, 
 190 
 
 Monte Rotondo, Tuscany, 216 
 Moon, effect of internal forces 
 
 on, 305 
 
 Mountains, all volcanoes not, 2 
 Mountain-chains, formation of, 
 
 291 
 all of recent date, 292 
 
 PHO 
 
 Mud streams at volcanoes, 30 
 
 Mud- volcanoes, formation of, 
 181, 182 
 
 Mull, dissected volcano of, 142-4, 
 tigs. 57, 58 
 
 Muscovite, absence of, from mo- 
 dern lavas, 268 
 
 MEBULAR hypothesis of La- 
 11 place, 325, 352 
 
 of Kant, 352 
 New Zealand, geysers of, 217 
 
 volcanoes of, 135 
 
 volcanic cones in, 79 
 
 Niedermendig, lava of, 103 
 NORDENSKIOLD, Prof essor, cited, 
 318 
 
 OBSERVATORY on Vesuvius, 
 24, 37 
 
 on Etna, 37 
 Obsidian, 59 
 Oceans, depth of, in volcanic 
 
 areas, 242 
 Oceanic islands, volcanoes in, 
 
 228 
 
 OLIVER, Capt. S. P., 92 
 Oldhamite, 314 
 
 Outlines of Vesuvius, 87, fig. 17 
 Ovifak, iron-masses of, 319 
 Oxidation of materials of globe, 
 
 324 
 Oxygen, proportion in lavas, 47 
 
 PACIFIC, volcanoes in, 229 
 
 PALMIEKI, Professor, cited, 
 25, 37 
 
 Papandayang, eruption of, 169 
 
 Papin's digester, nature of ac- 
 tion in, 22 
 
 Parasitic cones, formation of, 161, 
 162, fig. 61 
 
 Pele's Hair, 71 
 
 Perlitic structure, 109 
 
 PHILLIPS, Mr. J. A., cited, 220 
 
 Phonolites, 50, 59 
 
378 
 
 INDEX. 
 
 PHO 
 
 Phonolite- volcanoes, 126 
 
 Photograph of Vesuvian eruption, 
 24, fig. 5 
 
 < Pine-tree ' appendage of Vesu- 
 vius, 29 
 
 Pitchstones, porphyritic, 60 
 
 Plateaux formed of lava-sheets, 
 270 
 
 Pliny, Elder, death of, 7 
 
 Plombieres, hot springs of, 147 
 
 Plutonic rocks, 61 
 
 Pompeii, nature of materials 
 covering, 117 
 
 Ponza Islands, 39 
 
 Ponza, sections in, 131, 132, figs. 
 51, 52 
 
 Porphyrites, 263 
 
 Porphyritic pitchstones, 60 
 
 Potentially liquid rock, 250 
 
 Pre- Cambrian volcanoes of Brit- 
 ish Islands. 274 
 
 Presence of water in lavas, 353 
 
 Pressure under which crystals 
 were formed, 65 
 
 Prcdazzo, ancient volcano of, 165, 
 fig. 67 
 
 Propylites, 199 
 
 Pseudo-dykes, 119 
 
 Pumice, how formed, 68 
 
 cause of white colour of, 71 
 
 floating on ocean, 73 
 
 on ocean-beds, 73 
 Pumice-cones, 154 
 
 Puy de Pariou, Auvergne, 193, 
 
 194, figs. 82, 83 
 Puzzolana, 89 
 
 "DAIN, accompanying volcanic 
 " outbursts, 30 
 Rate of movement of lava- 
 streams, 97 
 
 RATH, Professor VOM, 72 
 Red clay of ocean-beds, 74 
 Red Mountains, Skye, 144 
 Reservoirs beneath vo 1 canoes, 
 
 145 
 
 REYER, Dr. ED., experiments of, 
 125, 160 
 
 SIL 
 
 Revkjanes, eruption off, in 1783, 
 
 102 
 
 Rhyolites, 50, 59 
 RICHTHOFEN, Vox, cited, 198, 
 
 199, 200, 205 
 Rocca Monfina, 178, fig. 77 
 
 , 204 
 
 Rock-masses, movements of, 288 
 Rocky Mountains, 103 
 volcanoes of, 201 
 Rotomahana, sinter-terraces of, 
 
 185 
 Ropy -lavas, 98, fig. 24 
 
 i section in, 132, fig. 53 
 Sandwich Islands, lavas of, 
 
 125 
 San Sebastiano, destruction of, 
 
 26 
 San Stephana, section in, 131, 
 
 fig. 50 
 Santorin, 42 
 Sarcoui, Grand Puy of, 126, fig. 
 
 44 
 
 Sciarra del fuoco, 13 
 Scoria, how formed, 68, 70 
 Scoria-cones, altered by acid gas, 
 
 155 
 
 breached, 156 
 
 characters of, 153 
 
 preservation of, 155 
 
 red colour of, 154 
 Scoria-cone in Vesuvius, 122, fig. 
 
 39 
 Scoria-cone near Auckland, N. Z., 
 
 165, fig. 66 
 
 SCHMIDT, referred to, 153 
 Schreibersite, 314 
 SCROPE, Mr. POULETT, cited, 5, 
 
 69, 106, 135, 198, 205, 212, 238, 
 
 289 
 Sea of Azof, mud-volcanoes of, 
 
 182 
 
 SECCHI, Father, cited, 362 
 Shiant Isles, 105 
 Silica, presence in lavas, 47 
 Silicates in lavas, 47 
 Silicon, proportion in lavas, 47 
 
INDEX. 
 
 379 
 
 SIL 
 
 Siliceous sinter, deposits of, 220 
 Silver, spitting of, 355 
 SILVBSTEI, Professor, cited, 230 
 Similarity of lavas of different 
 
 ages, 260 
 
 Sinter-cones, forms of, 183, fig. 79 
 Skye, dissected volcano of, 144 
 Slags, compared with lavas, 46 
 SMITH, LAWRENCE, cited, 320 
 Smoke, appearance of, due to 
 
 steam, 2 
 Snowdon, 274 
 Solar prominences, 364-366, figs. 
 
 92, 93, 94 
 
 Solfatara of Naples, 214 
 Solfatara-stage of volcanoes, 215 
 Somma, 133 
 
 crater-ring of, 83 
 
 SORBY, Mr. H. C., referred to, 
 
 59, 252 
 SPALLANZANI, early researches 
 
 of, 4 
 
 observations on Stromboli, 8 
 
 cited, 39, 357 
 
 Specific gravities of lavas, 49 
 
 of glassy and crystalline 
 
 rocks, 59 
 
 Spectroscope in vulcanology, 41 
 
 Spectrum-analysis, results of, 
 311 
 
 Specular-iron, deposited on lava- 
 streams, 110 
 
 Sperenberg, boring of, 341 
 
 SpTicerulites, 54, Frontispiece 
 
 Sporadosiderites, 316 
 
 Stability of crust of globe, 326 
 
 Staffa, Isle of, 106 
 
 STEENSTRUP, cited, 319 
 
 Steam-engine compared to vol- 
 cano, 8 
 
 Sttam, emitted by lava of Vesu- 
 vius, 27 
 
 STERNBERG, referred to, 113 
 
 St. Kilda, 181 
 
 STOKES, Professor, 65 
 
 St. Paul, Island of, 180 
 
 Stromboli, 42, 158 
 
 apertures at bottom of crater, 
 15 
 
 TEN 
 Stromboli, appearances in cratel 
 
 of, 16 
 at night, 10 
 
 compared with Vesuvius, 23 
 
 crater of, 13 
 
 dependence of eruptions on 
 atmospheric conditions, 34 
 
 eruption of, 14, fig. 4 
 
 general features of, 11 
 
 map of, 11, fig. 2 
 
 observations by Spallanzani, 8 
 
 resemblance to flashing light, 
 10 
 
 section of, 13, fig. 3 
 
 soundings around, 12 
 
 vapour-cloud above, 9 
 
 violent eruptions of, 23 
 Strombolian stage, 23 
 Stufas, nature of, 217 
 Submarine volcanoes, 179 
 Subterranean forces, beneficial 
 
 effects of, 303 
 
 Subsidence in centre of volca- 
 noes, 165 
 
 Sulphur, absorption of water by 
 molten, 356 
 
 deposited on lava- streams, 110 
 
 how formed at volcanoes, 18 
 
 not the cause of volcanic out- 
 bursts, 18 
 
 Surfaces of lava-streams, 97-99, 
 
 figs. 22, 23 
 Sun-spots, 361-363, figs. 89, 90, 
 
 91 
 
 Syenite, 59 
 Syssiderites, 316 
 SZABO, Professor, cited, 199 
 
 rFACHYLYTE, 59 
 
 -L Tertiary volcanoes of British 
 Islands, 276 
 
 Terraces, sinter- and travertine- 
 formation of, 185, fig. 80 
 
 Temperature, increase in deeper 
 parts of earth's crust, 335 
 
 rate of increase in different 
 areas, 340 
 
 Teneriffe, 44, 151 
 
380 
 
 INDEX. 
 
 TEN 
 Teneriffe, 178, fig. 77 
 
 peak of, 175, fig. 73 
 Tenon-and-mortise structure in 
 
 basaltic columns, 107 
 Theodosius and Vulcano, 3 
 Thunder, accompanying volcanic 
 
 outbursts, 28 
 Trachytes, 49, 50, 59 
 Trap-rocks, origin of, 241 
 Trass, 90 
 Travertine or Tibur-stonc, 184 
 
 deposits of, 220 
 
 Triassic volcanoes of British 
 Islands, 275 
 
 Tridymite deposited on lava- 
 streams, 110 
 
 Troilite, 314 
 
 TEOOST, cited, 355 
 
 Tufa, or tuff, 90 
 
 Tuff-cones, characters of, 157 
 
 denudation of, 157, 158 
 Typhon, fable of, 3 
 
 ULTRA-BASIC lavas, 50, 56 
 rocks, 317 
 
 V AL DEL BOVE, Etna, 133, 
 V 180, 209 
 
 dykes in, 134. fig. 55 
 Vapour- cloud over Vesuvius, 26, 
 29 
 
 Stromboli, 9 
 
 Ventotienne, Island of, section 
 
 at, 130, fig. 49 
 Vesuvius, 37 
 
 changes in form of, 81 
 
 compared with Stromboli, 23 
 
 crater of in 1756, 84, fig. 14 
 
 of in 1767, 85, fig. 15 
 
 of in 1822, 82, fig. 13 
 
 of in 1843, 86, fig. 16 
 
 detonations at, 26 
 
 early history of, 83 
 
 eruption of year 79, 84 
 
 of 1822, 69 
 
 of April 1872, 24 
 
 of October 1822, 24 
 
 VOL 
 
 Vesuvius, ejected blocks of, 45 
 
 first eruption of, 7 
 
 form of, 166 
 
 fossils of, 45 
 
 growth of cone of, 80 
 
 history of, 204 
 
 last eruption of, 7 
 
 - lava-stream of 1855, 101 
 
 lava-streams of 1858, 1872, 
 97 
 
 lavas of, 104 
 
 minerals of, 46 
 ejected at, 149 
 
 observatory on, 24, 37 
 
 outlines of, 87 
 
 pine-tree appendage of, 29 
 
 scoria-cones in lava, 122 
 on lava of 1855, 153 
 
 steam emitted by lava of, 27 
 
 vapour-cloud over, 26, 29 
 Vesuvian stage, 23 
 
 eruption, pliotograpTi of, 24, 
 fig. 5 
 
 Viscid lavas of Lipari Islands, 
 
 94-96 
 Vitreous lavas, devitrification of, 
 
 259 
 Volcanic action, laws of, 32 
 
 bombs, 70, 71 
 
 cycles, nature of, 221, 222 
 duration of, 223 
 
 cc-nes, internal structure of, 
 115-122 
 
 experi-tnentaliUiistrationoJ 
 
 formation of, 120, fig. 37 
 
 limits to height of, 166 
 
 form of, 152 
 
 dimensions of, 152 
 
 irregular development of, 
 90 
 
 slopes of sides of, 91 
 
 composed of ejected rock- 
 fragments, 156 
 
 curved slopes of, 156 
 
 debris on sea-bottom, 240 
 
 dust, fineness of, 69 
 
 districts, areas of upheaval 
 245 
 
 ejections, alteration of, 258 
 
INDEX. 
 
 381 
 
 VOL 
 
 Volcanic eruptions, compared to 
 ebullition, 19, 20 
 
 forces, compensate for denu- 
 dation, 283 
 
 intensity at former periods, 
 
 278 
 
 ~ - necessity for action of, 285 
 shifting of from one area 
 
 to another, 277 
 
 mountains, origin of conical 
 forms, 89 
 
 mode of growth, 89 
 
 phenomena of the past simi- 
 lar to those at present, 273 
 
 product s, order of appearance 
 of, 198, 199 
 
 Volcanic rocks, 61 
 
 similarity of ancient and 
 
 modern, 253 
 Volcano, origin of name, 3 
 
 craters of, 167 
 
 Island of. See Vulcano 
 
 compared to steam-engine, 8 
 Volcanoes, blocks, ejected from, 
 
 45 
 built up of ejected fragments, 
 
 74 
 destruction caused by, 281 
 
 dissected by denudation, 115, 
 139 
 
 erroneous ideas concerning, 1 
 
 ejection of different materials 
 from, 205 
 
 known to ancients, 3 
 
 life-history of, 186 
 
 number of, 224, 225 
 
 of Africa, 227 
 
 of America, 236 
 
 of Asia, 236 
 
 of Bohemia, 126 
 
 of Central Asia, 236 
 
 of Central Pacific, 236 
 
 of Europe, 227 
 
 of Hungary, 126 
 
 position in relation to moun- 
 tain chains, 243 
 popular ideas concerning, 1 
 reservoirs beneath, 145 
 
 ZEO 
 
 Volvic, lava of, 103 
 VOSE, cited, 346 
 Vulcan, forge of, 3 
 Vulcano, island of, 3, 158 
 Vulcano and Theodosius, 3 
 VuJcano, 178, fig. 77 
 
 and Vulcanello, vierv of, 43, 
 fig. 6 
 
 chemical deposits at, 44 
 
 eruption in 1786, 43 
 
 - in 1873, 43 
 
 lava-stream in, 95, fig. 19 
 
 - 103, fig. 27 
 
 pla of, 195, fig. 85 
 
 section of volcanic we in, 116, 
 fig. 35 
 
 section in, 129 
 
 shifting of centre of eruption 
 in, 196 
 
 Vulcanello, craters of, 197, fig. 86 
 Vulcanology, origin of the 
 science, 4 
 
 earliest treatise on, 5 
 
 WALFKRDIN, M., cited, 340 
 Water in lavas, 353 
 
 penetration through rocks, 358 
 
 presence of in lavas, 102 
 
 and saline solutions in cavi- 
 ties of crystals, 63 
 
 WERNER, cited, 201 
 
 Western Isles of Scotland, 102, 
 139, 142 
 
 volcanoes of, 212 
 
 WHYMPER, Mr., 69 
 
 WOODWARD, Mr., experiments 
 of, 119 
 
 Wrekin, ancient volcanic rocks 
 of, 259 
 
 'OUNG, Professor, cited, 365 
 
 7EOLITES, formation of, 160 
 
INTERNATIONAL SCIENTIFIC SERIES. 
 
 NOW READY. In 12mo, and bound in cloth. 
 
 1. FORMS OF WATER. By Prof. JOHN TYNDALL. $1.50. 
 
 2. PHYSICS AND POLITICS. By W. BAGEHOT. $1.50. 
 
 3. FOODS. By EDWARD SMITH, M. D., LL. B., F. R. S. $1.75. 
 
 4. MIND AND BODY. By ALEXANDER BAIN, LL. D. $1.50. 
 
 5. THE STUDY OF SOCIOLOGY. By HERBEKT SPENCER. $1.50. 
 
 6. THE NEW CHEMISTRY. By Prof. JOSIAH P. COOKE, Jr. $2.00. 
 
 7. THE CONSERVATION OF ENERGY. By Prof. BALFOUR STEWART. $1.50. 
 
 8. ANIMAL LOCOMOTION. By J. B. PETTIGKEW, M.D. $1.75. 
 
 9. RESPONSIBILITY IN MENTAL DISEASE. By H. MAUDSLEY, M.D. 
 
 $1.50. 
 
 10. THE SCIENCE OF LAW. By Prof. SHELDON AMOS. $1.75. 
 
 11. ANIMAL MECHANISM. By E. J. MAREY. 117 Illustrations. $1.75. 
 
 12. CONFLICT BETWEEN RELIGION AND SCIENCE. By J. W. DRAPER, 
 
 M.D. $1.75. 
 
 13. THE DOCTRINE OF DESCENT, AND DARWINISM. By Prof. O. SCHMIDT. 
 
 $1.50. 
 
 14. THE CHEMISTRY OF LIGHT AND PHOTOGRAPHY. By Dr. HERMANN 
 
 YOGEL. 100 Illustrations. $2.00. 
 
 15. FUNGI; their Nature, Influence, and Uses. s ByM. C. COOKE, LL.D. Edited 
 
 by M. J. BERKELEY. 109 Illustrations. $1.50. 
 
 16. THE LIFE AND GROWTH OF LANGUAGE. By Prof. W. I). W'HITNEY, of 
 
 Yale College. $1.50. 
 
 IT. MONEY AND THE MECHANISM OF EXCHANGE. By W. STANLEY 
 JEVONS, M. A., F. R. S. $1.75. 
 
 18. THE NATURE OF LIGHT. By Dr. E. LOMMEL. 88 Illustrations and a Plate 
 
 of Spectra in Chromo-lithography. $2.00. 
 
 19. ANIMAL PARASITES AND MESSMATES. By Prof. M. VAN BENEDEN. 
 
 83 Illustrations. $1.50. 
 
 20. ON FERMENTATIONS. By P. SCHUTZENBERGEK. $1.50. 
 
 21. THE FIVE SENSES OF MAN. By Prof. J. BERNSTEIN. $1.75. 
 
 22. THE THEORY OF SOUND IN ITS'RELATION TO MUSIC. By Prof. PIETRO 
 
 BLASERNA. Numerous Woodcuts. $1.50. 
 
 23. STUDIES IN SPECTRUM ANALYSIS. By J. N. LOCKYER. $2.50. 
 
 24. A HISTORY OF THE GROWTH OF THE STEAM-ENGINE. By ROBERT 
 
 H. THITRBTON, A. M., C. E. 163 Illustrations. $2.50. 
 
 25. EDUCATION AS A SCIENCE. By ALEX. BAIN, LL.D. $1.75. 
 
 26. MODERN CHROMATICS. By Prof. O. N. ROOD. 130 original Illustrations. $2.00. 
 
 27. THE HUMAN SPECIES. By A. DE QUATBEFAGES. *'2.00. 
 
 28. THE CRAYFISH: An Introduction to the Study of Zoology. By Prof. T. H. 
 
 HUXLEY. 82 Illustrations. $1.75. 
 
 29. THE ATOMIC THEORY. By AD. W URTZ, Membre de Plnstitut, etc. Trans- 
 
 lated by E. CLKMINSHAW. $1.50. 
 
 30. ANIMAL LIFE AS AFFECTED BY THE NATURAL CONDITIONS OF 
 
 EXISTENCE. By Prof. KARL SEMPER. $2.00. 
 
 31. SIGHT : An Exposition of the Principles of Monocular and Binocular Vision. By 
 
 JOSEPH LE CONTE, LL. D. With numerous Illustrations. $1.50. 
 
 32. GENERAL PHYSIOLOGY OF MUSCLES AND NERVES. By Dr. I. ROSEN- 
 
 THAL. With 75 Woodcuts. $1.50. 
 
 33 ILLUSIONS: A Psychological Study. By JAMES SULLY. $1.50. 
 34. THE SUN. By C. A. YOUNG, Ph.D., LL.D., Professor of Astronomy hi the 
 
 Collese of New Jersey. With numerous Illustrations. $2.00. 
 
 Any volume sent by mail, post-paid, on receipt of price. 
 
 NEW YORK: D. APPLETON & CO., 1, 3, & 5 BOND STREET. 
 
D. APPLETON & CO.'S PUBLICATIONS. 
 
 ILLUSIONS: A Psychological Study. By JAMES SULLY, author of "Sen- 
 sation and Intuition," etc. (" International Scientific Series.") 12mo, cloth. 
 Price, $1.50. 
 
 This volume takes a wide survey of the field of error, embracing in its view not 
 only the illusions commonly regarded as of the nature of mental aberrations or hallu- 
 cinations, but also other illusions arising from that capacity for error which belongs 
 essentially to rational human nature. The author has endeavored to keep to a strictly 
 scientific treatment that is to say, the description and classification of acknowledged 
 errors, and the exposition of them by a reference to their psychical and physical con- 
 ditions. 
 
 41 The latest number of Appletons 1 'International Scientific Series 'bears the title 
 of ' Illusions, 1 and is an interesting contribution by Mr. James Sully to the study of 
 mental pathology. The author's field of inquiry covers all the phenomena ot illusion 
 observed in sense-perception, in the introspection ot the mind's own feelings, in the 
 reading of others 1 feelings, in memory, and in belief. The author's conclusions are 
 often illustrated by concrete example or anecdote, and his general treatment of the 
 subject, while essentially scientific, is sufficiently clear and animated to attract the 
 general reader. 11 New York Sun. 
 
 "This is not a technical work, but one of wide popular interest, in the principles 
 and results of which every one is concerned. T ho illusions of perception of the senses 
 and of dreams are first considered, and then the author passes to the illusions of intro- 
 spection, errors of insight, illusions of memory, and illusions of belief. The work is a 
 noteworthy contribution to the original progress of thought, and may be relied upon 
 as representing the present state of knowledge on the important subject to which it 
 is devoted.' 1 Popular Science Monthly. 
 
 A SELECTION FROM THE LETTERS OF MADAME DE REMU- 
 
 SAT. 1804 1814. Edited by her Grandson, PAUL DE EEMUSAT, Senator. 
 Uniform with "Memoirs of Madame de Kemusat," 1S02--1SOS. 12mo, cloth. 
 Price, $1.25. 
 
 " 'A Selection from the Letters of Madame de Remusat to her Husband and Son 1 
 has been published by the App!etons. Coming closely upon the fascinating memoirs 
 of that lady, they possess the same interest, and will add to the reader's knowledge 
 of social and political life in France in the days of the first Napoleon. They deal, of 
 course, largely with family matters, but for all that we get vivid glimpses of public 
 life, and, from Madame de Kemusat's intimate acquaintance with political affairs, we 
 can pick up here and there hints which do not appear in French history, and yet are 
 absolutely necessary to a thorough understanding of it." Boston Ecening Tran- 
 script. 
 
 THE ART OF SPEECH. Vol. II. Studies in Eloquence and Logic. By L. 
 T. TOWNSKND, D. D., Professor in Boston University, and Dean of Chautauqua 
 School of Theology. Uniform with Vol. I. Studies in Poetry and Prose. 18mo, 
 cloth. Price, 60 .cents. 
 
 CONTENTS OF VOL. II: Part I, Studies in Eloquence: Introductory; History of 
 Eloquence; Life and Character of Demosthenes ; Oration on the Crown; Inferences; 
 Inferences (continued)', Inferences (continued): Inferences (concluded). Part II. 
 Studies in Logic: Introductory; Argumentation; Classification; Practical Observa- 
 tions. Supplemental Notes. 
 
 For sale by all booksellers: or sent by mail, post-paid, on receipt of price. 
 
 D. APPLETON & CO., Publishers, 
 
 1, 3, & 5 BOND STREET, NEW YOEK. 
 
D. APPLETON & CO.'S PUBLICATIONS. 
 
 MILITARY HISTORY OF ULYSSES S. GRANT. From April, 1861, to 
 April, 1865. By General ADAM BADEAU, Aide-ae-Camp to the General-in- 
 Chief. Volumes II and III. This work is now complete in three octavo 
 volumes, containing over 2,000 page**, illustrated with a Steel Portrait and 
 thirty-three Maps. Price, in cloth, $12.00; sheep, $15.00; half morocco. 
 $20.00. Sold only by subscription. 
 
 RISE AND FALL OF THE CONFEDERATE GOVERNMENT. By 
 
 JEFFERSON DAVIS. Complete in two volumes, 8vo. Illustrated with Por- 
 traits of Mr. Davis, his Cabinet, Aides, and Generals, and with Maps and 
 Plans. Price, per volume, in cloth, $5.00; sheep, $6 00; half turkey. $7.00; 
 full turkey, $10.00. Sold by subscription only. 
 
 "Every impartial reader must recognize the ability \vith which it is com- 
 posed, the sincerity with which his opinions are held, and the good faith with 
 which they are set forth, and the value which it possesses as the authentic com- 
 mentary on the most momentous episode in the history of the United States 
 since their independence was acknowledged and their Constitution was framed." 
 London Athenaeum. 
 
 ANECDOTAL HISTORY OF THE BRITISH PARLIAMENT. From 
 the Earliest Periods to the Present Time, with Notices of Eminent Par- 
 liamentary Men and Examples of their Oratory. Compiled by G. H. JEN- 
 NINGS. Crown 8vo, cloth. Price, $2.50. 
 
 " As pleasant a companion for the leisure hours of a studious and thoughtful 
 man as anything in book-shape since Selden." London Telegraph. 
 
 YOUNG IRELAND. A Fragment of Irish History, 1840-1850. By the Hon. 
 Sir CHARLES GAVAN DUFFY, K. C. M. G. New cheap edition. 12mo, cloth. 
 Price, $1.50. 
 
 "Never did any book appear so opportunely. But, whenever it had ap- 
 peared, with so lucid and graphic a style, so large a knowledge of the Irish Ques- 
 tion, and so rtatesmanlike a grasp of its conditions, it would have been a book 
 of great mark." London Spectator. 
 
 A HISTORY OF GREECE. From the Earliest Times to the Present. By 
 T. T. TIMA-ZENIS. With Maps and Illustrations. Two volumes, 12mo, 
 cloth. Price, $3.50. 
 
 HISTORY OF HERODOTUS. An English Version, edited, with Copious 
 Notes and Appendices, by GEORGE RAWLINSON, M. A. With Maps and 
 Illustrations. New edition. In four volumes, 8vo, vellum cloth. Price, 
 $8.00. 
 
 THE DICTIONARY OF ROMAN AND GREEK ANTIQUITIES. 
 With nearly 2,000 Engravings on. Wood, from Ancient Originals, illustra- 
 tive of the Industrial Arts and Social Life of the Greeks and Romans. By 
 ANTHONY RICH, B. A. Crown 8vo, cloth. Price, $3.00. 
 This work is now offered at the above greatly reduced price. 
 
 D. APPLETON & CO., Publishers, 
 
 1, 3, & 5 BOND STREET, NEW YORK. 
 
D. APPLETON & CO.'S PUBLICATIONS. 
 
 THE LAND OF GILEAD. With Excursions in the Lebanon. By LATJ- 
 BENCE OLIPHANT. With Illustrations and Maps. Crown 8vo, cloth. Price, 
 $2.00. 
 
 "His journeys took him quite off the beaten tracks of tourists and archaeo- 
 logical explorers; he got an 'inside view,' so to call it, of native life and man- 
 ners ; he saw something of the wandering Bedouins ; and we know of no recent 
 book on Palestine which is really so instructive, from which the reader can de- 
 rive so large a fund of entertainment." Eclectic Magazine. 
 
 NEW YORK ILLUSTRATED. An entirely new edition of this work, ex- 
 tended to more than double the number of pages of previous editions \ and 
 including many new illustrations, making a most superb pictorial volume 
 descriptive of the great metropolis. W r ith new cover. Large 8vo. Price, 
 75 cents. 
 
 SOCIAL ETIQUETTE OF NEW YORK. New and enlarged edition. 
 Containing two additional chapters " Extended Visits," and "Customs 
 and Costumes at Theatres, Concerts, and Operas 1 ' with the chapter on 
 "Etiquette of Weddings" rewritten in accordance with the latest fashion- 
 able usage. 18mo, cloth, gilt. Price, $1.00. 
 
 A TREATISE ON THE DISEASES OF THE NERVOUS SYSTEM. 
 
 By WILLIAM A. HAMMOND, M.D., Surgeon-General U. S. Army (retired 
 list); Professor of Diseases of the Mind and Nervous System in the Uni- 
 versity of the City of New York. One volume, 8vo, 928 pages, with about 
 150 Illustrations. Price, in cloth, $5.00 ; sheep, $6.00. 
 
 This, the seventh edition of Dr. Hammond's well-known work, has been thor- 
 oughly revised, and enlarged by the addition of new chapters and of a section on 
 Diseases of the Sympathetic System. 
 
 The work has received the honor of a French translation by Dr. L^badie- 
 Lagrave, of Paris, and an Italian translation by Professor Diodato Borrelli of 
 the Royal University, is now going through the press at Naples. 
 
 A TEXT-BOOK OF PRACTICAL MEDICINE, with Particular Refer- 
 ence to Physiology and Pathological Anatomy. By Dr. FELIX VON NIE- 
 METER, Professor of Pathology and Therapeutics, Director of Medical 
 Clinic of the University of Tubingen. Translated from the eighth German 
 edition by GEORGE H. HUMPHREYS, M. D., and CHARLES E. HACKLEY, M. D. 
 New revised edition, with additions. Two volumes, 8vo, 767, 861 pages. 
 Price, in cloth, $9.00; in library leather, $11.00. 
 
 Since Dr. Niemeyer's death a new edition of his text-book has appeared in 
 Germany, edited by Dr. Eugene Seitz. This has been freely drawn upon in the 
 present revised American edition, and additions have been made from other 
 sources of a character calculated to render the work more useful to American 
 readers. 
 
 D. APPLETON & CO., Publishers, 
 
 1, 3, & 5 BOND STREET, NEW YORK. 
 
Scientific Publications. 
 
 GENERAL PHYSIOLOGY OF MUSCLES AND NERVES. By Dr. I. 
 
 ROSENTHAL, Professor of Physiology at the University of Erlangen. With 
 seventy-five Woodcuts. (" International Scientific Series.") 12mo, cloth, 
 
 $1.50. 
 
 "The attempt at a connected account of the general physiology of muscles 
 and nerves is, as far as I know, the first of its kind. The general data for this 
 branch of science have been gained only within the past thirty years." Extract 
 from Preface. 
 
 SIGHT : An Exposition of the Principles of Monocular and Binocular Vision 
 By JOSEPH LE CONTE, LL. 1)., author of "Elements of Geology"; "Re- 
 ligion and Science " ; and Professor of Geology and Natural History in the 
 University of California. Witli numerous Illustrations. 12mo, cloth, $1.50. 
 
 " It is pleasant to find an American book which can rank with the very best 
 of foreign works on this subject. Professor Le Conte has long been known as 
 an original investigator in this department ; all that he gives us is treated with 
 a master-hand.'" The Nation. 
 
 ANIMAL, LIFE, as affected by the Natural Conditious of Existence. By 
 KAKL SEMPER, Professor of the University of Wurzburg. With 2 Maps 
 and 106 Woodcuts, and Index. 12mo, cloth, $2.00. 
 
 " This is in many respects one of the most interesting contributions to 
 zoological literature which has appeared for some time." Nature. 
 
 THE ATOMIC THEORY. By AD. WUKTZ, Membre de Tlnstitut ; Doyen 
 Honoraire de la Faculte de Meclecine ; Professeur a la Faculte des Sciences 
 de Paris. Translated by E. CLEMINSHAW, M. A., F. C. S , F. I. C., Assist- 
 ant Master at Sherborne School. 12mo, cloth, $1.50. 
 
 " There was need for a book like this, which discusses the atomic theory both 
 in its historic evolution and in its present form. And perhaps no man of this 
 age could have been selected so able to perform the task in a masterly way as 
 the illustrious French chemist, Adolph Wnrtz. It is impossible to convey to the 
 reader, in a notice like this, any adequate idea of the scope, lucid in?tructiveness, 
 and scientific interest of Professor Wurtz' s bock. The modern problems of 
 chemistry, which are commonly so obscure from imperfect, exposition, are Ijere 
 made wonderfully clear and attractive." The Popular Science Monthly. 
 
 THE CRAYFISH. An Introduction to the Study of Zoology. By Professor 
 T. H. HUXLEY, F. R. S. With 82 Illustrations. 12mo, cloth, $1.75. 
 
 " Whoever will follow these pages, crayfish in hand, and will try to verify for 
 himself the statements which they contain, will find himself brought face to face 
 with all the trreat zoological questions which excite so lively an interest at the 
 present day." 
 
 ' The reader of this valuable monograph wi'l lay it down with a feeling of 
 wonder at the amount and variety of matter which has been got out of so seem- 
 ingly slight and unpretending a subject." Saturday Review. 
 
 D. APPLETON & CO., Publishers, 
 
 1, 3, & 5 BOND STREET, NEW YOKK. 
 
Scientific Publications. 
 
 THE HUMAN SPECIES. By A. DE QUATREFAGES, Professor of Anthro- 
 pology in the Museum of Natural History, Paris. 12mo, cloth, $2.00. 
 
 The work treats of the unity, origin, antiquity, and original localization of 
 the human species, peopling of the globe, acclimatization, primitive man, forma- 
 tion of the human races, fossil human races, present human races, and the physi- 
 cal and psychological characters of mankind. 
 
 MODERN CHROMATICS. With Applications to Art and Industry. With 
 130 Original Illustrations, and Frontispiece in Colors. By OGDEN N. ROOD 
 Professor of Physics in Columbia College. 12mo, cloth, $2.00. 
 
 "In this interesting book Professor Rood, who, as a distinguished Professor 
 of Physics in Columbia College, United States, must be accepted as a competent 
 authority on the branch of science of which he treats, deals briefly and succinctly 
 with what may be termed the scientific rationale of his subject. But the chief 
 value of his work is to be attributed to the fact that he is himself an accom- 
 plished artist as well as an authoritative expounder of science." Edinburgh 
 fieview, October, 1879, in an article on " The Philosophy of Coloi\" 
 
 EDUCATION AS A SCIENCE. By ALEXANDER BAIN, LL. D. 12mo, cloth, 
 
 $1.75. 
 
 " This work must be pronounced the most remarkable discussion of educa- 
 tional problems which has been published in our day. We do not hesitate to 
 bespeak for it the widest circulation and the most earnest attention. It should 
 be in the hands of every school-teacher and friend of education throughout the 
 land." New York Sun. 
 
 A HISTORY OF THE GROWTH OF THE STEAM-ENGINE. By 
 
 ROBERT H. THUKSTON, A.M., C. B., Professor of Mechanical Engineering 
 in the Stevens Institute of Technology, Hoboken, N. J., etc. With 163 
 Illustrations, including 15 Portraits. 12mo, cloth, $2.50. 
 
 " Professor Thurston almost exhausts his subject ; details of mechanism are 
 followed by interesting biographies of the more important inventors. If, as is 
 contended, the ?team-engine is the most important physical agent in civilizing 
 the world, its history is a desideratum, and the readers of the present work will 
 agree that it could have a no more amusing and intelligent historian than our 
 author." Boston Gazette, 
 
 STUDIES IN SPECTRUM ANALYSIS. By J.. NORMAN LOCKYER, F. R. 8., 
 
 Correspondent of the Institute of France, etc. With 60 Illustrations. 12mo, 
 
 cloth, $2.50. 
 
 "The study of spectrum analysis is one fraught with a peculiar fascination, 
 and some of the author's experiments are exceedingly picturesque in their re- 
 tults They are so lucidly described, too, that the reader keeps on, from page 
 to paae. never flawing in interest in the matter befoiv him, nor putting down 
 the book until the last page is reached." New York Evening Express. 
 
 D. APPLETON & CO., Publishers, 
 
 1, 3, & 5 BOND STREET, NEW YORK. 
 
Scientific Publications. 
 
 TEXT-BOOK OF SYSTEMATIC MINERALOGY. By HENRY BAUER- 
 MAN, F. G. S., Associate of the Koyal School of Mines. (New volume in 
 the "Text-Books of Science Series.") 16mo, cloth. Price, $2.50. 
 
 ANTHROPOLOGY : An Introduction to the Study of Man and Civilization. 
 By EDWARD B. TYLOR, D.C.L., F. R. S., author of "Primitive Culture," 
 "The Early History of Mankind," etc. With 78 Illustrations. 12mo. 
 With Index. Cloth, $2.00. 
 
 "Mr. Tylor's admirable little book certainly deserves the success with which 
 it will doubtless meet." Pall Mall Gazette, 
 
 SCIENTIFIC CULTURE, AND OTHER ESSAYS. By JOSEPH PARSONS 
 COOKE, Professor of Chemistry and Mineralogy in Harvard College. One 
 vol., square 16mo, cloth. Price, $1.00. 
 
 POPULAR LECTURES ON SCIENTIFIC SUBJECTS. By H. HELM- 
 HOLTZ, Professor of Physics at the University of Berlin. Second Series. 
 12mo, cloth, $1.50. 
 
 The favor with which the first series of Professor Helmholtz's lectures was 
 received justifle?, if a justification is needed, the publication of the present 
 volume. 
 
 THE POWER OF MOVEMENT IN PLANTS. By CHARLES DARWIN, 
 LL. D., F. R. S., assisted by FRANCIS DARWIN. With Illustrations. 12mo, 
 cloth, $2.00. 
 
 " Mr. Darwin's latest study of plant-life shows no abatement of his power of 
 work or his habits of fresh and original observation. We have learned to expect 
 from him at intervals, never much prolonged, the results of special research in 
 some by-patli or other subordinated to the main course of the biological system 
 associated with his name; and it has been an unfailing source of interest to see 
 the central ideas of the evolution and the continuity of life developed in detail 
 through a series of special treatises, each wellnigh exhaustive of the materials 
 available for its subject." Saturday Review. 
 
 A PHYSICAL TREATISE ON ELECTRICITY AND MAGNETISM. 
 
 By J. E. H. GORDON, B. A., Assistant Secretary of the British Association. 
 With about 200 full-page and other Illustrations. 2 vols., 8vo, cloth, $7.00. 
 
 41 We welcome most heartily Mr. Gordon's valuable contribution to the experimen- 
 tal side of the science. It at once takes its place among the books with which every 
 investigator and every teacher who goes beyond the merest rudiments must needs 
 equip himself. There is certainly no book in English we think there is none in any 
 other language which covers quite the same ground. It records the most recent ad- 
 vances in the experimental treatment of electrical problems, it describes with minute 
 carefulness the instruments and methods in use in physical laboratories, and is prodi- 
 gal of beautifully executed diagrams and drawings made to scale." London Times. 
 
 D. APPLETON & CO., Publishers, 
 
 1, 3, & 5 BOND STREET, NEW YOBK. 
 
Elementary Works on Mechanical and Physical Science, 
 
 FORMING A SERIES OF 
 
 TEXT-BOOKS OF SCIENCE, 
 
 Adapted, for the Use of Artisans and Students 
 in Publie and Science Schools. 
 
 
 
 FULLY ILLUSTRATED SIZE, 16mo. 
 
 VOLUMES ALREADY PUBLISHED: 
 
 The Elements of Mechanism. By Professor T. M. GOODEVE, M. A. Cloth, 
 $1.50. 
 
 Metals : Their Properties and Treatment. By Professor C. L. BLOXAM. Cloth, 
 $1.50. 
 
 Introduction to the Study of Inorganic Chemistry. By W. A. MIL- 
 LER, M. D., D. C. L., LL. D. Cloth, $1.50. 
 
 Theory of Heat. By Professor J. C. MAXWELL, M. A., LL.D. Cloth. $1.50. 
 
 The Strength cf Materials and Structures. By J. ANDERSON, C. E., 
 LL.D., F.E.S.E. Cloth, $1.50. 
 
 Electricity and Magnetism. By Professor F. JENKIN, F. E. SS. L. & E., 
 M. I. C. E. Cloth, $1.50. 
 
 Workshop Appliances, including Machine-Tools used by Engineers. By C. P. 
 B. SHELLEY, C. E. Cloth, $1.50. 
 
 Principles of Mechanics. By Professor T. M. GOODEVE, M. A. Cloth, $1.50. 
 
 Introduction to the Study of Organic Chemistry. By Professor H. E. 
 ARMSTRONG, Ph. D.,F. C. S. Cloth, $1.50. 
 
 Qualitative Chemical Analysis and Laboratory Practice. By Pro- 
 lessor T. E. THORPE, Ph. D., F. E. S. E., and M. M. P. MUIR, F. E. S. E. Cloth. 
 $1.50. 
 
 Telegraphy. By W. H. PREECE, C. E., and J. SIVEWRIGHT, M. A. Clotk $1.50. 
 
 Railway Appliances. By J. W. BARRY, C.E. Cloth, $1.50. 
 
 The Art of Electro-Metallurgy. By G. GORE, LL. D., F. E. S. Cloth, $2.50. 
 
 Introduction to the Study of Chemical Philosophy. By W. A. TIL- 
 DEN, D. Sc. Lend., F. C. S. Cloth, $1.50. 
 
 The Elements of Machine Design. By Professor W. C. UNWIN, C. E. 
 Cloth, $1.50. 
 
 Treatise on Photography. By Captain W. DE WIVELESLIE ABNEY, F. E. S. 
 Cloth, $1.50. 
 
 The Study of Rocks ; an Elementary Text -book of Petrology. By FRANK EXIT- 
 LEY, F. G. S., of H. M. Geological Survey. Cloth, $1.75. 
 
 Elements of Astronomy. By E. S. BALI,. Professor of Astronomy in the Uni- 
 versity of Dublin. With Illustrations. Cloth, $2.25. 
 
 Text-book of Systematic Mineralogy. By HENRY BAUERMAN, F. G. S.,' 
 Associate of the Eoyal Scnool of Mines. Cloth, $2.50. 
 (OTHER VOLUMES IN PREPARATION.) 
 
 "It would be a difficult task to single out another list of text-books on the same 
 or collateral subjects in our language which could be compared with them, either in 
 regard to quality and price, or that are so well fitted for the instruction of engineering 
 students, or for students generally in our public and science schools." London Ex- 
 aminer. 
 
 For sale by all booksellers ; or any volume sent by mail, post-paid, on receipt ol 
 price. 
 
 D. APPLETON & CO., Publishers, 1, 3, & 5 Bond St., New York. 
 
P RIMERS 
 
 IN SCIENCE, HISTORY, AND LITERATURE. 
 
 18mo. . . . Flexible cloth, 45 cents each. 
 
 L Edited by Professors HUXLEY, ROSCOE, and BALFOUR 
 STEWART. 
 
 SCIENCE PRIMERS. 
 
 Introductory. .... ,.,'. T. H. HUXLEY. 
 
 Chemistry H. E. ROSCOE. 
 
 Physics BALFOUR STEWART. 
 
 Physical Geography..., , A. GEIKIE. 
 
 Geology A. GEIKIE. 
 
 Physiology , M. FOSTER. 
 
 Astronomy c J. N. LOCKYER. 
 
 Botany J. D. HOOKER. 
 
 Logic , W. S. JEVONS. 
 
 Inventional Geometry..., .., W. G. SPENCER. 
 
 Pianoforte FRANKLIN TAYLOR. 
 
 Political Economy W. S. JEVONS. 
 
 JI. Edited by J. R. GREEN", M. A., Examiner in the School of Modern 
 History at Oxford. 
 
 HISTORY PRIMERS. 
 
 Greece C.A.FYFFE. 
 
 Rome M. CREIGHTON. 
 
 Europe ....E. A. FREEMAN. 
 
 Old Greek Life J. P. M AHAFFY. 
 
 Roman Antiquities 4 A. S. WILKINS. 
 
 Geography GEORGE GROVE. 
 
 III. Edited by J. R. GREEN, M. A. 
 
 LITERATURE PRIMERS. 
 
 English Grammar , R. MORRIS. 
 
 English Literature STOPFORD A. BROOKE. 
 
 Philology J. PEILE. 
 
 Classical Geography , M. F. TOZER. 
 
 Shakespeare E. DOWDEN. 
 
 Studies in Bryant J. ALDEN. 
 
 Greek Literature R. C. JEBB. 
 
 English Grammar Exercises R. MORRIS. 
 
 Homer W. E. GLADSTONE. 
 
 English Composition J. NICHOL. 
 
 (Others in preparation.) 
 
 The object of these primers is to convey information in such a manner as 
 to make it both intelligible and interesting to very young pupils, and so to dis- 
 cipline their minds a? to incline them to more systematic after-studies. In the 
 Science Series some simple experiments have been devised, leading up to the 
 chief truths of each science. By this means the pupil's interest is excited, and 
 the memory is impressed so as to retain without difficulty the facts brought 
 pnder observation. The woodcut? which illustrate these primers serve the 
 aoie purpose, embellishing and explaining the text at the same time. 
 
 D. A PPL ETON &> CO., Publishers, New York. 
 
APPLETONS' SCHOOL READERS, 
 
 Consisting of Five Books. 
 
 BY 
 
 WM. T. HARRIS, LL, D,, A. J. RICKOFF, A. M., MARK BAILEY, A. M. } 
 
 Superintendent of Schools, Superintendent of Instruction, Instructor in Elocution, 
 
 St. Louis, Mo. Cleveland, 0. Yale College. 
 
 4PPLETONS' FIRST READER Child's Quarto, 90 pages. 
 
 APPLETONS' SECOND READER 12mo, 142 " 
 
 APPLETONS' THIRD READER 12mo, 214 
 
 APPLETONS' FOURTH READER 12mo, 248 " 
 
 APPLETONS' FIFTH READER 12mo, 4T1 
 
 CHIEF MERITS. 
 
 These Readers, while avoiding extremes and one-sided tendencies, combine 
 into one harmonious whole the several results desirable to be attained in a series 
 of school reading-books. These include good pictorial illustrations, a combina- 
 tion of the word and phonic methods, careful grading, drill on the peculiar com- 
 binations of letters that represent vowel-sounds, correct spelling, exercises well 
 arranged for the pupil's preparation by himself (so that he shall learn the great 
 lessons of self-help, self-dependence, the habit of application), exercises that 
 develop a practical command of correct forms of expression, good literary taste, 
 close critical power of thought, and ability to interpret the entire meaning of the 
 language of others. 
 
 THE AUTHORS. 
 
 The high rank which the authors have attained in the educational field and 
 their long and successful experience in practical school-work especially fit them 
 for the preparation of text-books that will embody all the best elements of modern 
 educative ideas. In the schools of St. Louis and Cleveland, over which two of 
 them have long presided, the subject of reading has received more than usual at- 
 tention, and with results that have established for them a wide reputation for 
 superior elocutionary discipline and accomplishments. Feeling the need of a 
 series of reading-books harmonizing in all respects with the modes of instruc- 
 tion growing out of their long tentative work, they have carefully prepared these 
 volumes in the belief that the special features enumerated will commend them 
 to practical teachers everywhere. 
 
 Of Professor Bailey, Instructor of Elocution in Yale College, it is needless to 
 speak, for he is known throughout the Union as being without a peer in his pro- 
 fession. Hie methods make natural, not mechanical readers. 
 
 D. APPLETON & CO., PUBLISHEBS, 1, 3, & 5 BOND STREET, NEW YORK, 
 
tti 
 
RETURN EARTH SCIENCES LIBRARY 
 TO* 642-2997 
 
 LOAN PERIOD 1 
 1 MONTH 
 
 2 
 
 3 
 
 4 
 
 5 
 
 6 
 
 ALL BOOKS MAY BE RECALLED AFTER 7 DAYS 
 
 Books needed for class reserve are subject to immediate recall 
 
 DUE AS STAMPED BELOW 
 
 FORM NO. DD8 
 
 UNIVERSITY OF CALIFORNIA, BERKELEY 
 BERKELEY, CA 94720